I would get one from the store because…
1: They would have a great range and selection
2: They would have healthy Betta’s
3: You would save them from being in a small container.

As with any type of critter, you always want a healthy pet, I would think about going to a private breeder, when you go to his/her home to pick out your critter, look at the quality of the tank and see how clean the area is. Make sure the fish are healthy in every tank that is set up in thier home, you’ll know what kind of breeder they are from seeing how they themselves live and how they take care of thier pets. If everything looks great, then buy from this person. Pet-Stores don’t care about the living conditions of breeders, as long as they can sell, they could care less.

Get them from a breeder. That way, you know you’re getting quality. Bettas especially have different tail types – if you spawn, for instance, 2 Veiltails from your local pet store, the babies will be nearly impossible to sell, as the market is saturated with them. If you spawn 2 Halfmoons with impressive color and form, then more people will be interested in the babies.

You can find some absolutely stunning bettas on Aquabid.com, which is like eBay for fish. You can also post on a forum like Ultimate Bettas – someone may have a pair they’d be willing to send to you.

Good luck!

I would advise getting your breeding stock from a breeder, but make sure you look into how they care for their fish before you purchase any. Breeders tend to take MUCH better care of their bettas than a pet store does. In fact, most of the bettas you see in pet stores are suffering from ammonia burn from being in a tiny jar with no water changes for far too long. I always get my bettas from pet stores becuase I like to rescue them from poor living conditions, so I don’t have a site I could recommend, sorry! Good Luck though.