There is no such thing as a 'small' breeding tank when it comes to bettas. Bettas are territorial and require a 'territory' to be able to breed in, which need to be more than about two cups (they average size of those 'betta condos' and 'spawn tanks'). Afterall, it's not just sticking the female in with the eggs and letting her do the rest, the female will eat her own eggs. The male is the one that cares for the eggs and young in the first few days after the coupling and the female dropping her eggs. There must be sufficient space in the tank for the male to place the eggs in a bubblenest he has constructed - eggs that are allowed to fall tot he bottom of the tank will NOT hatch. And you must be able to get the female back out of the tank after the spawning, without disturbing the bubblenest, male, and eggs - this just really isn't possible in one of those tiny tanks. Sticking the two fish together in such a tiny space to get them to breed will not work, they will only attack each other. A 20 gallon long or greater is recommended if you intend upon breeding bettas. I suggest you try caring for your current pair of bettas bette rand researching the subject more thoroughly before attempting such a difficult endeavor as breeding them. If you haven't got a lot of space, then don't breed bettas, it's that simple.
I have at least one local pet store around my location that does buy bettas, Mr. Petman. I imagine if you go and speak with the store manager of your LFS you could find out what stores around you do and don't take bettas - although I'll say right now they generally won't pay much (they want wholesale) and they won't take them until they are generally at least 3.5-4 months old and are showing good finnage. So it will be neccessary for you to have your own grow-out containers for each and every male (leaving them together once you can spot they are male generally means the aggression stage is right around the corner and they will start fighting each other soon). You aren't going to make money at breeding bettas and selling the fry, the amount of time and money it costs to be able to raise the fry is far more than you will be able to sell them to the store for.