Amazon.com most of the time, some times Ebay but not nearly as much as Amazon, very well trust place to shop online IMO.

Gamestop is "OKAY" but there not the best anymore.

Flea markets.

In Italy the only cheap ones are budget editions, old games included with magazines and bargain bin ones. Good online offers are good value only spreading shipping fees amongst more games. When I decide to buy from abroad, I found that also Amazon.co.uk is good value if spreading shipping fees, with the added advantage that I can spread them on books too if I'm interested only in one game, otherwise I found good prices at www.play.com and www.gameplay.co.uk, the latter is at all effects inside the EU, so no duties, checkout price is what you pay, the former is based in Jersey, a Crown Dependency outside of EU, so purchases are subject to duties, but small purchases should fall within duty-free allowance (Googling around I found the allowance was recently increased in EU).

Pawn shops.

shopto

i usually get them from amazon UK when the euro is closer to the pound.

Walmart