i got the 3 gallon grow bags that came with my kit from http://www.htgsupply.com i love them. you can roll down the top if you want it shorter. holes on the bottom are kinda big but excellent for drainage, i throw down a layer of perlite on the bottom so not as much soil runs out. although its not really much anyway. i just went to menards and got those 10 or 12 inch treys for pots. also every once in a while i gently press around the sides to loosen up the soil a bit. much harder to do that with plastic pots. you should also be able to get them for extremely cheap. they are very sturdy for me.
my htg supply kit came with 12 3 gallon grow bags, and 3 bags of fox farm ocean forest potting soil. it was just enough to fill them all but not completely full. each bag has 1.5 cubic feet of soil so a total of 4.5 cubic feet was put into 12, 3 gallon grow bags.
sorry for delayed response. 50 of the 3 gallon grow bags would use about 12.5 bags of soil. so i would get 13. assuming each bag of soil is 1.5 cubic feet. personally i like to throw a layer of perlite on the bottom for better drainage but you dont have to. if you do im sure you could get away with 12 bags. however having the extra soil never hurts. heres my math below in fractions. the fraction on the left is soil bags. the fraction on the right is grow bags or pots. the 3 is how many bags of soil would be needed for 12 pots. X is how many bags of soil(1.5 ft³) you need for 50 pots or grow bags.
