There aren't many used media stores around my area unfortunately.
There is two game stores within a 35 mile radius and that is Gamestop and a mom & pop shop called A to Z Games. Unless I'm looking for something no older than the PS3 era, Gamestop is useless. A to Z is a great place to find games and the folks there are always getting "new" stuff in. You could literally go there every day and there are a bunch of new things to go through. Unfortunately, I rarely have the time to go there because when I get off work at 3:30, go home and get changed, it around 4. It's a half-hour drive from my house to the store and they close at 5. They are also closed on Sunday and Saturday I am usually busy with my daughter's soccer games, etc.
I've scoured the local Goodwill on several occasions. I sometimes find stuff, like sealed copies of Kirby's Epic Yarn a few weeks ago but most of the time, they are pretty bare for games. There are quite a few thrift shops scattered around the area that I have to check out. The Pawn shops around the area have pretty much done away with older games as well, so looking for anything pre-PS360 isn't going to pan out.
The other options I have are to go to the St. Louis area South County Mall. There are two stores there, V-Stock and Slackers. Both are stores that stock games, toys, movies, music, etc. Of the two, Slackers is significantly smaller and their prices are pretty high. I rarely buy something there. V-Stock on the other hand, I usually don't walk out of there without dropping at least $100. I find their prices pretty reasonable and the quality of their games is decent. For example, last time I was there, I got the original Ape Escape black label in very nice condition for $12.99.
When it's a game that I can't find anywhere else, it's an import or I absolutely want the best quality/condition possible, I usually turn to eBay. Sometimes it can be pricey and you have to really watch who you buy from to make sure your not getting screwed buying a "like-new" game that has rental stickers all over the disc and a dilapidated manual. I've been looking for a black label copy of Spyro Year of the Dragon and found one. I won it for $9.99 plus $2.50 s&h. Likewise, I've always wanted a black label copy of the first Crash Bandicoot which is always pricey and hard to find. Nabbed one freshly listed for $18.99 with free shipping. I actually gambled on this one as their was only one picture and a brief description. But knowing how fast they get snagged up, I jumped. The game arrived. The disc has a few teeny-tiny scratches (which I expected from the description) and the manual's only damage is a small spine bend and "dings" where the little notches are on the case that keep the manual from just sliding out on it's own.
Sorry for rambling on, but that's where I get the majority of my goodies.