If you must use feeders, grow your own. That would make guppies or some easily-bred egglayer such as convicts the only feasible source. You should NEVER, and I repeat NEVER buy feeder fish from any pet shop.
As has been noted above and cannot be repeated too often, feeders are a prime vector for the introduction of parasites and diseases into your tanks. When commercially produced, these fish are bred and kept in substandard conditions all the way from their point of origin into the holding tanks at your LFS (just look at how overcrowded they are and recognize what this does to their water conditions and their susceptibilty to disease, for example). Feeding a sick fish to another fish is by far the most effective way to transmit that disease. For a non-aquatic parallel, remember that the way cows get BSE (Mad Cow disease) is by eating food made up in part of ground up pieces of other infected cows.
Quarantining store-bought fish really won't work, either, because you can easily miss the symptoms of various bacterial or parasitic problems (until they show up in your prize specimen fish, that is).
This board has gone around and around on this subject in the past, and has more than a few cases where someone who was particularly adamant about the need to use feeders and the lack of risk therefrom during a discussion would come back a month or two later to post a "...I didn't think it would happen to me, but you guys were right..." comment.
The moral of the story is that the odds will almost always catch up with you if you don't heed the advice. Using feeders from an LFS is like playing bacterial Russian roulette with your fish.
So get a big tank and raise your own. Although with all of the foods commercially available today, it seems like a lot of trouble (and tankspace that could be put to other, better uses) for nothing.