u/Apprehensive-Dust-70

Hi I’m interested in getting isopods or springtails but idk where to get them, how many to get, which one is better or should I have both or only one, or how to put them into my tank, do I just place them in and let them lose or is there a safer way of doing it? Also am I correct in assuming they eat poop and leftover food?

I don’t have many card stores in my area, I haven’t checked them yet but when I get a chance I wanna but I was curious if like Walmart, target, or somewhere similar would sell them. Or do I have to buy them online?