There are virtually no foreign vets in regular practice in BKK. The main reason is that one needs to obtain a Thai vet license by taking the board exams in Thai to work in a normal practice or clinic. Some foreigners work under the umbrella of international NGO's. You might want to contact, for example, WSPA who have their Asia headquarters in BKK. The vets there are primarily allowed to work in an advisory capacity on specifically sanctioned programs.
The leading professional organization is the Veterinary Practitioners Association of Thailand. I wouldn't expect too much encouragement from them though, as they are jealously guarding their turf and prevent even foreign voluntee