I from California's sf bay area. It's seems like snes games are the most abundant. I never noticed this until I started really looking for genesis games. The local shops have racks of snes and even nintendo but at 2 stores there genesis selection was a little larger then a shoebox. Our craigslist is about the same. I also do the flea market thing pretty regularly and hit garage sales when I can.
I could probably break it down like this...(1 being the most abundant.)
2. Gameboy advance
8. Game gear
9. sega cd
10. Turbo graphic 16
11. Neo geo (were talkin like 5 games I seen in the past year!)
12. Master system
Oddly since I've been into retro games in the last few years I can't even remember seeing a 32x game.
As for pre Nes systems(atari ect.) I haven't seen any in some time.
Is that just here? I wonder if everywhere else it's more even?
What's your local list like?