I wouldn't reccomend petsmart. There are several sites where you can order quality show guppies for breeding purposes. The only reason to buy at a lfs is if you want to develop your own line or something like that.
Try the guppy store section on the International Fancy Guppy Association website, www.ifga.org.
Gary and Tim Mousseau, and Luke Roebuck both sell some very fine strains and both have produced recent show winners.