Last summer when I had just started getting back into the old stuff, I searched CL for NES and found a yard sale about an hour out from me that promised to have "all systems from 2600 to Gamecube". I ran up there only looking for a NES, and go that for $25 with 2 controllers, all hookups and Mario/Duck Hunt. The woman had no less than 10 complete systems of EACH system from 2600 to Gamecube. Kicking myself for not picking up a SNES when I was there so I could have gotten in on it earlier.
After that, it was just thrift shops/flea markets with some success. This guy on CL sold me Mega Man 2 and 3 for $3 a piece and told me about this fabled guy in a run down flea market. I showed up and he had BINS of cart only games, numbering in the thousands. After a few trips I had taken the good things from him, but I imagine he might have rebuilt his stock by now.
Other than that, yard sales and garage sales have been a bust, but I'm being optimistic for this summer.
RE: CRT's, yeah I get that, but I just want a nice little 20' or so with S-Video that I can sit on my entertainment center when I play, and then toss in the closet when I'm done.