Look for a fish with clear eyes (no clouding), good finnage (dont get feather tails or rose tails, theyre pretty but sickly) and look for a good seller. There are many US sellers, which ship cheaper than Thai sellers with just as good of fish.
Examples of rose/feather tails:
Poor suckers tend to not be as healthy.
Also, dont fall for a lot of the crap that AB sellers try and pull off... You might notice it from time to time-- SPECIAL DIAMOND EYE BETTA! is actually a poor blind betta, but theyre trying to get you interested... Get my drift?
Halfmoons and halfmoon plakats tend to be the most popular fish. Halfmoons do tend to have tail problems, though- since they have huge tails.
HM plakats dont suffer the same problem, but like with any short finned fish, they should be given at least 2 gallons of heated tank.