The Volunteer Work Thailand website offers a list of non-profit organization working in Thailand that are looking for international volunteers. It is a fairly comprehensive list and includes some basic information about each organization, such as what they do, location, length of stay, what’s provided.
Another place to search for job, internships and volunteer opportunities in the non-profit sector is idealist.org. The section for Thailand offers some good leads, but many are related to volunteer teaching in Thailand. It is worth checking out though and hopefully more organizations in Thailand will use this resource so there will be a greater variety of opportunities listed.