There are a ton of these up here in Chiang Mai. It seems to be a bike that has finally gotten some recognition of classic status. I have asked around for them myself and a bike that has been well restored, with licence and registration seems to run from 15-20,000. Maybe a trip up north would be a productive place to start looking.
Can't say anything about Bangkok though, I know that does not help that much but maybe it will give you a reference point.