Nobody here mentioned bike co-ops. There are only a couple in the LA area, but they're also a good source for used bikes, and if you want to fix up whatever you end up buying.

And by the way, I live on my bike, so I'm poorer than you. Would never dream of putting it out there for someone to donate a bike to me. Even tho there are countless bikes wasting away in backyards, garages, etc that were bought, used once or twice and now sit unattended.