Turtles from pet stores are usually very sick since half, if not most the time, the idiots who run the store have no idea how to properly take care of these animals.
Adopting, and buying from good, reputable pet stores, or breeders online is the way to go (though there are some that you should stay away from, anyone buying from a breeder online MUST try to do a little background check on the person/company) places like
www.turtletimes.com/forums and
www.turtleforum.com have forums about breeders and peoples experience with some of them, if you got ripped off, scammed, animal arrived severely unhealthy, or dead, post about it and warn people not to buy from them, this helps people when they are deciding which breeder to buy from, 'the good, the bad and the ugly'
However, whenever looking for a Red-eared slider, adoption is THE best way to go since the species that most of us on this forum keep our considered 'cockroaches' to the rest of the turtle community.