If you are determined to specifically experiment with legal highs, hawaiian baby woodrose seeds are probably the least nasty source for the lsa type alkaloids (whats in morning glory seeds etc) but any of the seeds containing those alkaloids also contain other fun chemicals that pretty much turn your stomach and guts into high powered pumps that will violently expel your gastrointestinal contents out both ends.
http://en.wikipedia.org/wiki/Ergometrine the side effects are really unsavory. You can definitely trip really hard off of them, but at what cost...
Due to being unable to tell the exact quantities of alkaloids in a given amount of material, it is difficult to find a dose that minimizes the negative effects and maximizes the positive. 90% chance you'll be unable to appreciate the trip at all due to puking your guts out
One of my worst drug experiences ever was with several HBWR seeds and some syrian rue, I was praying to the porcelain gods for mercy all night long, curled up miserable and twitching on the cold tile floor barely holding on to my conscious mind enough to remind myself that eventually morning and sobriety would come, and there would be more to life than endless vomiting.
Morning glory seeds are cheap, "effective" and readily available, but after trying several different methods of ingestion including low doses combined with herbal maois to potentiate, cold and hot water extraction, polar/nonpolar defatting type methods, just straight up eating the seeds...there is just no way to get around the awful side effects. The side effects are caused by the same or similarly structured chemicals as are making you trip so there is no way around it. Also frequent repeat doses can cause vasoconstriction issues in your fingers and toes, and I have personally experienced this. This vasoconstriction can also cause nasty headaches, powerful nausea, and other gastrointestinal complaints. You will definitely trip, but 9 times out of 10 I was totally unable to enjoy the experience for the side effects.
You could also try ayahuasca type dmt containing plants, mimosa hostilis root bark for example and syrian rue or your choice of herbal maoi, but I have had poor luck with sourcing decent quality material, and you will still also spend alot of time vomiting which I am not really into. Are you picking up on the theme here yet?
San Pedro or Peruvian torch cactus will do the trick, but I have had no luck getting high off of similar looking species of cactus acquired at a home depot type store or garden store. From what I understand there are dozens of similar looking species with no psychoactive effects whatsoever and even botanists have a hard time telling them all apart without a microscope at hand, so if you choose that option, dont waste your money, order from a reputable supplier online and order more than you think you'll need. You could grow it in your garden but it takes a really long time (a year+) and if you can afford it you'll probably be better off just ordering enough to eat on the spot. (and either way it would still be cheaper if you just want to trip to buy some shrooms) The cactus tastes and has a texture like bitter, very slimy/mucousy cucumber. There are teks online for doing an acid/base extraction of psychoactive material if you don't wanna eat a bowl of slime chunks, or drink a mucous milkshake. (ps. you will probably want to throw up some more if you drink that but you need to keep it down till you start to feel effects) If you paid attention in highschool chemistry you should be able to follow the tek.
http://www.erowid.org/plants/cacti/cacti_chemistry2.shtml ... Also poke around on google, you will find some pictoral examples. Someone else will have to point you to another supplier- Bouncingbearbotanicals.com was my vendor of choice but they no longer carry such cactus at the moment.
Salvia is pretty easy to grow, but you'll have to order a cutting online unless you are lucky enough to have nearby headshop that carries live plants. The plant rarely produces seeds so don't be tricked into buying fake seeds somewhere. Salvia is not cold-hardy so you'll need to house it indoors under lights in the winter. Substantial amounts of leaves are needed to make a potent enough tincture or a potent enough extract/leaf mixture to produce profound psychoactive effects- and if you succeed in making a 20 or 40x extract (acetone works well as a solvent for this) it smokes like burning plastic, and you've still got to hold in for as long as you can a couple of _really harsh_ lungfuls of smoke to get anywhere. A butane torch lighter and a bong are highly recommended for this. Don't be afraid to put some serious heat on the plant material, it takes a bit to vaporize the salvinorin. Pending you managed to get enough psychoactive material into your bloodstream in the relatively narrow dose time window, you will find yourself, twitching, drooling, and dropping your pipe on the floor as your consciousness is quickly twisted into a strange vision, sometimes a full on dreamstate kind of hallucination where for 5-15 minutes you are not even conscious whatsoever of the world around you and are completely absorbed in another world.
More likely though, you will have failed to get quite a powerful enough dose the first 3-4 times you try (you can try again every hour or two if you really want, tolerance fades that quickly) you will just get a really strange feeling, your speech gets very slurred, movements twitchy, thoughts seem to struggle to hold structure, focusing on anything is difficult and the images of things you are looking at sort of fall apart and sounds echo, kinda as if your consciousness was shattered into disjointed pieces or something, difficult to describe. If you get this feeling, you know you are close to a potent enough dose to have a breakthrough trip. I have heard people describe lower doses of salvia than this to just make them feel profoundly stupid, uncoordinated in movement and thought, as if all their senses were crudely blunted. To me only the breakthrough experience was worthwhile, I found the other effects when I failed to get a sufficient dose to be unpleasant. But hey, at least it doesnt make you puke!
Salvia I would really recommend doing with a sitter, if for nothing else than to keep your pipe from burning a hole in your carpet, clothes, or person if you drop it.
Kava Kava and Kratom definitely both work, but they are more of a relaxant/sedative kind of experience and the effects are pretty mild. Kratom resin tastes and smells amazing though, and gives a mild sort of opiate like high for a few minutes with a brief hint of euphoria when smoked. I have not had a chance to do anything but eat kava kava extract gelcaps from healthfood store which were mildly sedative if eaten by the handful. If anyone has actually ordered fresh kava root from an ethnobotanical shop I'd be curious to know how it worked for you.
Blue lotus for me was barely beyond placebo, I thought I maybe got a hint of a warm fuzzy relaxation feeling but it was hard to tell. Tasty tea regardless though and very pretty.
Most legal/herbal highs come with either a heavy price in negative side effects, dont work consistently, or sometimes at all, and end up costing more than is worth your while.
Abuseable OTC substances are not without risk either- I am part of the small percentage of the population that has a bad reaction to DXM - what was a pleasant moderate dose for a friend caused me hours of intense burning pain and blackouts. I spent the night in a tub full of cold water to keep my temperature down.
Whatever you choose to do, do it safely, test subthreshold doses of substances to make sure you tolerate them well, and have trustworthy peeps on call to help you. For all my negativity YMMV- some people really enjoy their ethnobotanicals.