You need six months validity to travel internationally or will need to obtain an ETD from your respective Embassy to leave Thailand if this is the case.
If I understand the post correctly....if you Google "International Law Firms Thailand" there are several firms that will offer Notarial and Will writing services. 🙂
If it's of any help.... straight from the Thai Immigration page, with notable exceptions mentioned by Stuart already; make of it what you will:-
"According to The Immigration Act,B.E.2522, the foreigner who has received a temporary stay permit and stayed in the Kingdom of Thailand over 90 days must notify his/her residence to immigration officer every 90 days."