Yes, they do sell official piano sheet music - I can't remember exactly where on the internet I bought them but they're called 'piano collections (insert game here)'.
I own two of them personally (IX and XIII) and the sheet music for 'those who fight' (i.e. VII's battle theme). My personal favourites are 'rose of may' and 'melodies of life' from IX. They're definitely worth getting IMO.

P.S. They're in Japanese (well mine are at least - I don't know if there are English versions out there)

P.S.S They're also arrangements so they're generally a little different from how you remember them. I'd recommend looking up some videos on youtube first