Some Ma & Pa pet shop will take/buy them, some will give you store credit and some will not.....Store credit can vary from a dime each to five dollars each, the Betta will either be cupped for re-sale or used for feeders-depending on the quality, tail type, age, etc.......
The big box type pet shop rarely will take home spawned fish due to them having contracts from fish farms
Craiglist, aquabid, ebay, fish forum-but you will need to know how to ship
Friends, family, neighbors
Local aquarium clubs- are nearly always a good way to rid yourself of home spawned fish and great to join and be a part of.....
Usually breeding fish especially Bettas will cost you more than you will make unless you have good connections or willing to ship and even then you have to spawn lots of Bettas and other species of fish, inverts and plants to make a profit when dealing locally
You breed/spawn for the pure joy of doing it and cull hard at a young age to limit your numbers to start until you find a good method to re-home/sell, join the local fish club and go to meetings, go to local pet shops and talk face-to-face with the owner or setup a large cichlid tank