You can always adopt them out online, with the adopters usually willing to pay shipping cost. Or if they're local pick up.
You can keep numbers down by culling fry at a young age that have curved spines or missing fins(when they're really young i cull by feeding to larger fish). If you have quality parents, one or two of the babies may even be ok enough to sell for $5-$10
I know my local independent pet store will take any healthy bettas i have left over(trade for some fish food or something), as they don't have a supplier for bettas . Maybe you can look locally too.
If you want to start working on your own line, or breeding more then once as a hobby you can cull deformed ones early then undesirables later on(poor fins or unwanted coloring). This will leave you with only fish that you plan to breed to work on your line and such.
Culling older fish isn't necessary killing them, you can adopt them to non-breeding homes.