Welcome to the sickness of betta adiction
Firstly, betta from any major chain can be a risk, because you don't know how old they are -- however, some of the best fish can be found in walmarts/petsmart/petco's you just have to be able to pick the good ones, the ones you want are going to be the most lively ones, moving around, maybe the one who's flaring at the other guys in the cup the most.
If you want some of the more fancy type of betta (Crown Tail, Half Moon, Delta, Super Delta, Plakat) check the yellow pages and call around to some local pet/fish shops and ask what kind of betta they have, sometimes you luck out and have a decent store around
Tank size... the bigger the better, contrary to what the big chains and evil evil tank makers will tell you, betta actualy like space, and like to swim! one gallon is the minimum, but feel free to go beyond that, they will thank you!
Heaters are good if you have a tank to suit it, bettas like to be warm, they will be the most active, and build the best bubble nests when they are warm and comfy
Also, silk plants, not plastic plants, betta have very fragile fins and will rip them up good on plastic plants. so go silk
Good luck and welcome to the world of bettas
