Parts for big bikes can be hard to find, but this place may be able to help you:
Jiew Charoen Yon Bike Parts Imports
This outfit imports 2nd hand parts and bikes from Japan. We saw about 50 big bikes in the shop, plus about 50 engines and bikes in various stages of disassembly. Most of the bikes were big choppers, with a good number of other types of medium and big bikes as well. They told us that they import the parts directly from Japan and sell all over Thailand.
There are also a large number of very used looking big tyres lying around. They are probably useful in an emergency only.
If you don't speak Thai ring one of the mobile numbers, as they speak English.
081-4097878 & 081-8581550
The location is in Doembang Nangbuat (~ 45km N of Suphanburi) on h'way 3032, about 100m from the jct. with h'way 340 on the S side.
GPS: N14.80842 E100.11862