McAmbros, there are several sites to have a look: Bahtsold, mocyc (as NickyBKK mentioned), thaisecondhand all (ad) dot com.
Just search the Northern forum and contact Richie at Richco's, he might be able to help as he is the local dealer for HD up here. Should find this information under a thread: Motorbike shops in Northern Thailand. Good luck with your search. Rgds, Franz