Most chain stores will not take fish from breeders as they have contracts with the wholesellers, along with insurance with them, etc.
Local fish stores that are sold and ran by individuals may if they don't have a good supplier.
You would be surprised at what people will want.. if they like the fish then it doesn't matter how good they are (healthy is the main importance).
These fish came from a mix spawn of mine
- wasn't expecting much from them and ended up with these guys. Not what you term as "grade A", but I sold them all within 24 hrs for $10 each.. they are prettier in life though, but with the bad quality pictures and them being so young and not fully developed they still sold.
So don't think you won't/can't sell the fish.. it's possible :)
As for conservation purposes then you would want to look for groups who do work to save the different betta species that are becoming extinct in the wild due to being over caught. You don't really give them to pet stores, but there are groups that ask for you to give them half the spawn to help other breeders out, and you can sell the other half, etc. Similar to this system which I support
But yeah, expect to have a lot of bettas hanging around while you sell them.. sometimes it's quick, sometimes it takes a bit. You can also look to see if you have a wholeseller close by to take the fish you can't sell. I was lucky and found one about 30 minutes from me - he takes 50% of the profits from selling the fish to stores/individuals, but it's at least something rather than culling or giving out for free (which most LFS will take the fish for free or store trade).
This, sadly, is why a lot of breeders will only keep the best of the best - as unless you have a source, it can be difficult to rehome many bettas.