I've fed nightcrawlers from Walmart. I'd be a little worried about what might be in the soils of a commercial worm farm. I wouldn't sweat it if you just need small quantities, but if you need a lot of worms anyway I'd just set up a couple of 30-50gallon plastic sweater boxes with European nightcrawlers. I'd put 500 in each one. You can switch which one you're feeding out of periodically to make it less likely that you completely deplete your supply. Unfortunately, I did manage to deplete my supplies and now have to reorder, but since they reproduce pretty well it's still an excellent value.