When mine get big enough and start getting colours, after I pick the prettiest ones I want to keep I intend to just ring around a few shops and ask if anyone wants them for free. I don't expect anything for them. I'd just rather give them away then do something awful like have to cull them because I have no more room.
If you're having problems with them giving birth all the time and you're being overrun, get a fish in your tank that will eat fry. I only get fry when I deliberately have the mother give birth in a trap/net - my other fish eat them too quickly otherwise. I don't believe people who say fry will survive in a well planted tank at all. Mine is well planted - one night a platy will be fat and full of little eyeballs - the next day she's not, and not a fry to be seen in the tank! I'm pretty sure my pearl gouramis are the main culprits.
Good luck