III and IV's mobile versions are ports of the 3DS one, and you can find III on steam as well. The other mobile versions are probably ports (or at least close to) the GBA/PSP versions which was what I played and they are ok, but they are quite different from what you're used to (especially if you started with FFXIII!). I personally hated FFIII, but I have to say both III and IV remakes are very good. You can fidn FFIX on PSN, it's the PS1 version since it's never been remade.
The title you'll have most troubles finding I think would be FFXII, which is only for PS2. Hopefully they will remaster it for PS3/PS4 but no plans so far.

As a personal advice, if you haven't played them yet I would suggest you try out the tactics series which is amazing (and of course XIII-2 and Lightning Returns).