In Cincinnati, we don't have any "thrift stores" specific to computer parts, but there are a couple of normal Goodwills, Salvation Army, and St Vincent's that do get some. Usually it's the larger, more warehouse-looking ones.
I bought the 21" Trinitron that's in my cab from one. I've also picked up a 19" for $25. The worst part is dealing with the people that work there. While I can understand that they want to get as much as they can out of this stuff, no-one there has any clue what any of it is or does. Sometimes it works in my favor, sometimes it doesn't. It's not uncommon to see an old 386 with a 5-1/4" floppy marked $50. The 21" was marked $25, but was being sold as a pair with a 667mHz celeron HP system which was marked $75. I couldn't convince them to sell the monitor seperately (even though it was originally sitting on a different shelf). Luckily it was half-price day for electronics, so for $50 I got both. Even though I had to buy them together as a complete computer system, I was told it would be $5 more to get a keyboard and mouse with it.
It's extremely hit or miss. It's common to find decent speakers, numerous gamepads, and these days there have been a lot of steering wheels and flight yokes (picked up a Sidewinder Force Feedback wheel for $8). I've been seeing nicer monitors in the last year or so as I'm guessing people are jumping to flat-panels.
I'm still paranoid when buying monitors. Out of all computer equipment, they have the most problems that can't be fixed. Trying to test one out at the store can be difficult many times too. On the bright side, if it doesn't work out at least the money's going to charity.