Plants yellowing then dieing?

sounds like they need nitrogen, but that is just a guess with the info provided.

Where do i get that?

I got a pic look at the bottom and see the yellow. https://www.rollitup.org/marijuana-pics/index.php?n=950

anywhere that sells plant food. it will have three #'s on it. 1-2-3.
some thing like that but different #'s. you want the first # to be the biggest. this # represents the amount of nitrogen. when they start to flower you will have to switch nutes. for flowering you want the last 2 #'s to be bigger than the first. follow the feeding instruction on the package. it is usually 1 tablespoon per gallon.

yeah, nitrogen should help. don't overdo it. it takes a few days to see results.

they need nitrogen (plantfood only a little bit at a time) and the Ph Is too high