Are there any other sources besides eBay or Craigslist to find used audio equipment? I am looking for a used Yamaha single cassette deck under $20 and when I asked a couple of neighbors, I've received sarcastic responses like I'm living in the stone age. Have you all had luck at garage sales (stopping at random ones when out driving) and is there an ability to test out the equipment at a garage sale?
You could try putting in a wanted in CL, or local Freecycle and see what might show up. Cassette players at garage sales are hit and miss, you'd best search CL garage sale listings or local estate sale listings for "cassette" to narrow down your search and driving - its very inefficient to drive around randomly for specialized stuff like this. Search for electronics (sorta generic terms for stereo) or stereo, tape, tape deck, etc. You can use Estatesales.net for your area.
As for testing, bring a set of earphones or headphones with 1/4" stereo jack or adapter for the mini-plug if that's what you have, and a few pre-recorded cassettes that you don't mind might get lost or chewed up - they're for testing only. If you can get the unit plugged in, you can check for basic FF/rewind, stop, play, and whether any signal goes to the meters, or to the headphone jack, which will hopefully tell you if the electronics are working and the alignment is roughly ok. Of course, the big thing is the mechanical mechanism and whether the tape/capstan/rollers work ok, and whether the take up reels work. Intermittent or scratchy volume, so long as you get some on each channel, might clean up with DeOxit cleaning to controls.
As for under $20, yeah you might find decks for that, but you'll have less likelihood that they work, unless someone is just getting rid of stuff and they just want it out, regardless of whether it works. Sure, testing may help weed stuff out, but really good decks by good makers are worth getting and repairing, but that assumes you have some type of budget for that purpose, IF you want a superior high fidelity deck.
As mentioned, US Audiomart will have stuff but its mostly higher dollar, but a want ad might shake out something.