Thrift stores are the cheapest bet (think out-of-the-way Mom and Pop Christian Ministry type thrift stores, not Goodwill).
Stopped by one recently and asked if they had any old keyboards in the back. The employee pointed me outside to a shed behind the building. There was literally a pile of old IBM Model Ms laying behind the place, had been hit by rain several times. Took a garbage sack of them home, let them dry out and everything was fine (there was even an old IBM model 5150, but my wife wasn't about to let me add that to the PC graveyard in the office).
The annoying bit is that so many of them are AT instead of PS/2 (good luck even finding PS/2 ports on modern hardware, now it is just 4 USB ports in the back).