Not being facetious Sir, but try a google, we sourced belts for st4000 this way, less than 10 Euro each.
If its not listed, find a specialist belt supllier and email them.
Hope that helps
Interesting - I have also just bought a replacement belt for my Autohelm 3000. Even though I used several contacts in the engineering trade, they all said it was a non-standard belt!
So I bought a proper Autohelm belt(D038) via Fox's in Ipswich for a reasonable £9.50(if my memory serves me right) with only a 3 day wait.
I did try a specialist supplier who would make to order but much more expensive!.
Interesting that the Autohelm site mentioned in a previous post gives a price of $36 !! Makes a change to be much cheaper in ££'s.