It's unlikely you'll have any more luck finding a compatible freehub (if such exists) than the complete wheel. The traditional method of upgrading a 7 speed is known as "8 of 9 on 7." You can Google it, but essentially 8 cogs of a 9 speed cassette will fit on your 7 speed freehub, no dropout spacing changes. Will require 9 speed shifter. I've done this on a road bike but I had all the parts already and wanted brifters instead of the 7 speed downtube shifters. Otherwise, I can't see a compelling reason to change from 7 speed.