It's been my personal experience that a lot of the LFS's bettas really aren't that old, at least none of the ones I bought weren't interested in breeding. I've bred 2 male bettas from petshops and 3 females.
The real important thing, I think, is to get 2 males because some just aren't made to be fathers. I would also get 3 females. It has been my experience that bettas form "pairs". Once my male bettas decided to breed with a certain female, they didn't want to breed with any of the other females if they had a choice. Vice versa for the females.
It's not a big deal if they are mutts or not because petshops are only going to pay you so much for bettas. Most, I don't think, would care if they are from a quality strain or not.
HTH