Good that you are thinking ahead on a very important step...
I am one that believes that everyone should spawn Bettas at least once, great experience that can lead to different and more difficult species
An average spawn for a new hobbyist is usually about 30 fry to adult stage or point that you have to cup the males
I would talk with the owners at your local ma & pa type pet shop and see if they have any interest in any Betta offspring you spawned, don't expect much for them if anything, most will give a store credit.
Be aware that sometimes they will be used as feeders- especially if they are mutts.......
Friends and family
Online...at tropical fish forms that have buy & sell section, however, shipping will be the big cost to either you or the buyer....not to mention the cost of shipping boxes, bags...etc......