Hey Tony,
I see you'e from BKK so I wouldn't know of any shops down there. What I can tell ya is most of the little shops on the street selling alternators & starters have little workshops where they're rebuilding them. I look for ones with a decent work area, hopefully competent staff, and ask to have them tested. At the very least rebuild the solenoid. All those copper parts in there start wearing early on and it seems many auto starter parts here are exact replacements for the ones in our bigger bikes. I just had mine done at a shop in Lamphun and all's well with it. Like many things here though, it may be hit & miss to find a shop you'e comfortable with.
Good luck!