I get my bikes on craigslist and garage sales.
Although it might not work the super-warm parts of the world, but in NC (where we have something resembling winter) I've found the prices much lower in winter and the deals remain on CL longer than in the summer.
In my area I think we have a craigslist-good-samaritan who occasionally contacts people saying their $50 bike is worth more. I have seen a number of ads jump in price by $100 shortly after being posted. I guess it could be a jump based on lots of emails requesting to buy the bike. Not sure.