I responded from my LFS, but they get most of them from a local breeder.
A few tips on picking a betta that have really worked well for me. Don't buy one that is being kept in a jar/cup. Many stores will have them spread out in tanks, or even have the HOT boxes that allow them to have 3 bettas in one tank, but kept apart with dividers. This is much preferable, since the water is less likely to be polluted. Bettas that can swim around the entire tank are, IMO, healthier, because they get more exercise.
Spend some time watching the fish. Don't get one just because you like it's color; make sure the fish is alert, flares a bit, and has healthy finnage. Never get a betta that is lying on the bottom of the tank/cup/jar. Also, the smaller the fish, the younger it probably is, so the longer it will live with you.
HTH