It depends on what games you want. Amazon is still selling all the FF games (except 7..wow) BRAND NEW and EXTREMELY CHEAP. Go check it out if you don't believe me, FF9 is $15 with free shipping.
I used to get games at Gamestop all the time, but they stopped carrying PSX games. I haven't bought any in a while because of that.
I've gotten a few harder to find games on eBay (like Rally Cross 1 & 2). eBay is great because the prices can be super cheap, I think all the PSX games I've gotten had higher shipping fees than the cost of what I paid for the game.
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Dedicated to mediocrity and repetition since 1994.