I just replaced the belt in my twin famicom, and by accident while looking for a replacement on ebay I came across some handy info:
the fds belt was a standard part for 3" disk drives, I bought some amstrad ones and they work great:)
the title of the listing I bought from is: Amstrad CPC 6128 & 6128 Plus Disk Drive Belts x2
an an excerpt from the listing:
For The Following Computers
Amstrad CPC 6128
Amstrad 6128 Plus
Amstrad PCW 8256 & 8512
also listed in the photo is the Sinclair Spectrum +3 and Amstrad FD1
these two belts together cost less than buying one with disk system in the title, so you can definitely save money by searching for belts for these computers instead of just searching disk system.