One -- Not everything has to be available on current hardware. Ebay exists.
Two -- Emulators exist, and FF6's SNES version is available on the Nintendo eShop and stuff as well. It's around.
The GBA version is pretty good, but it features a polarizing (technically better, but perhaps with less soul) translation and the music is quite a bit worse. If you don't have access to a SNES, or one of the Wii family of consoles it's the next most desirable version though, as the PS1 version is bad on a multitude of other levels.
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GBA:
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