I've had some good luck lately visiting some local Goodwills/private thrift shops in my area. I spend roughly 3 or 4 hours a week doing it, and tend to get good results fairly often. These pics comprise my finds from roughly the past 3 weeks or so (and roughly a dozen trips to various thrifts).
I paid between $4 and $6 for all of the Xbox/PS2/Gamecube/DS games. Highlights include a black label copy of Pikmin 2, which I found at an out-of-the way Goodwill. Workers there said a woman had cleaned out a bunch of Gamecube games an hour prior. Ironically, she probably missed the most valuable game in the lot. Paid $5 for it. Also found Phantasy Star Online, Jedi Knight: Jedi Academy, Mario Kart: Double Dash, and Star Wars Battlefront I and II for $6 each from a local thrift. Every game pictured was complete and in solid shape, some were even minty. Kept Mario Kart, Phantasy Star, Jedi Knight, and will flip the rest.
Paid $1 each for most of the PC/PS1 games pictured here, but did pay $4 each for Mega Man Legends and Monster Rancher 2. Duke Nukem CIB was $7, and the N64 and SNES controller were $3 each (both in great shape). Luigi's Mansion guide was 50 cents. Japanese PSP game AI Go was $5. Pokemon Gold (still saves) was $6. Highlight here is Jurassic Park: Operation Genesis for PC, worth about $50. Keeping Duke Nukem, since there is a special little corner in my heart for CIB SNES/N64 games. Did not picture a copy of Legend of Legaia for PS1 (purchased for $4), since I already sold it.
Overall, I hit 2 of my main stores about 3 times per week in the evening, and then just try to check in on various Goodwills/thrifts if I happen to be in the area. I use a smartphone (which I did not purchase just for game hunting) to look up the odd title that I'm not sure about, but at this point I look at 95% of titles and know whether or not it's a winner at a given price point. Definitely takes time to build up that level of knowledge, though. I have a decent job, and do all of this because I enjoy the thrill of the hunt. Good luck!