You can get pretty much any brand of kelp it doesn't really matter; I use kelp4less.com brand only because it was the cheapest I found at the time; buying in bulk and during promos/sales saves $$$. Down to earth products are good too but any dried kelp seaweed is fine even if you go to the beach and collect it yourself. The best worm castings are directly from a worms ass in a composting bin but again any brand will work. Unco brand is ez to find online; they work ok but bagged EWC is never as good as fresh.
You never have to be deadly accurate with organic amendments much unlike bottled nutrients. For top dressing I would mix up enough to cover the entire top layer of soil maybe about an inch & 1/2 deep in each of your containers. You only need a few tsp of kelp per container but you can always add as much EWC as you can fit. It won't burn your plants.
Having all this on hand will make brewing AACTs a snap. Molasses, kelp, and EWC makes a very good all purpose AACT when bubbled for 36+ hours. Give a tea every 2-3 weeks and push in a couple jobes spikes. Should keep them green through to harvest time.