Been doing this a number of years, and this is what it looks like from my perspective.
It's pretty easy to find NES, N64, GB/C, PS2, Xbox, and Atari 2600 - it varies, but you can find the quality stuff almost as often as the junk. It's somewhat more difficult to find PS1, Genesis, GG, SNES or GameCube - the thing is with those is when you find something it's always junk games or just the console. It's near impossible to find SMS, Saturn, Dreamcast, or other Atari - you can't find these at all, even the crappy stuff never turns up.
Some people seem surprised that I have a hard time finding PS1. I think N64 must have did much better here than PS1 (and that is how I always remembered it). I built my N64 collection in two short years of flea markets, while I had to go online, or to video game stores to buy many PS1 games becasue I never could find anything that wasn't Madden or Rugrats just floating around at a flea market, pawn shop, or thrift store.