I love www.aquabid.com

You can get extremely high quality Bettas/pairs for great prices, from great breeders. Just be sure to check the seller's reviews, and make sure you order from a seller in the US. You can often get beautiful Bettas for as little as $10, with $10-20 shipping.

www.bettatalk.com

Sells awesome Bettas, Faith is so nice! It can get very expensive, I recommend BT Bettas for breeders.