I breed red wigglers for my fish and have never had any problems. I actually started with red wigglers from my back yard. I keep them in soil and add vegetables that aren't used to be eaten and have never had a problem, The Doctor.
red wigglers are often called trout worms.. as already said.. NIghtcrawlers are hard to propigate with much luck. I like to buy mine for a local bait shop in the summer. and in the winter I stock up and keep them in a tote in the basement. you'de need multiple large bins to keep a constant supply of nightcrawlers on hand. depending on how many you want/need. I supprt the lil' private shops in our area. But you can get them at most petsmarts ( for 5$ here.. where the bait shop is 1/2 that )
Most my fish will spit/regurgitate bad worms. ( it happens ) So as long as the worms are health looking and squirmy you shouldn't have much issue with buying them most places. Just obviousely avoid those colored/scented ones you see every now and then.