A little late to the party but was on vacation and didn't take my laptop; phone only. A real PITA for posting.
Make sure you understand the difference between an importer and a breeder. I see so many on this Forum and FB using the terms interchangeably. An importer gets his or her Betta at anywhere from 3-6 months old and doesn't know parents whereas a breeder knows the parents and usually sells at four months.
I buy from two people: One is an importer on FB and another both imports and breeds. Each has lovely, healthy Betta.
The importer bred Betta for years but no longer has the enormous amount of time it takes to do it right; so he imports from Thailand and Vietnam. He knows his breeders.
If you don't intend to breed then pet store Betta are perfectly acceptable. Before I stopped going to pet stores in toto
I saw a lot of lovely Betta in pet stores I would have been proud to own. So do not hesitate to buy one....or two....or three
As far as swimming, almost all of my long-fins are exaggerated and haven't had one with a swimming problem. But I go against the "Betta don't like current" belief and use adjustable HOB filters. I increase the filter output as the Betta gains strength and can hold up those fins.
However, inability swim well does happen no matter what the owner attempts to help. And purchasing a young Betta with "normal" finnage doesn't mean its fins won't continue to grow until they become incredibly heavy.