If it's for a charity, maybe contact websites like BandCamp and see if they have a way for artists to "donate" music for your cause? There are a couple of those sorts of "pay what you think it's worth" sites out there, for charitable work I would think you could swing some kind of permission to buy music cheaply (or have the artist donate it) and allow you to put it on those players. It's not like you're going to be sharing it outside of this program or otherwise redistributing it...