Try and find some copies of the Guardian Weekly. It's a weekly compilation of news from the Guardian newspaper out of the UK and it's directed at expats. About once a month they have a section on aid and development work, with a section of ads (jobs and schools) devoted to NGOs and development work, much like what you're looking for.

Even if nothing is posted for Thailand, you might get some leads on agencies you could approach about the type of job you want.
Cheers,
Bev