You haven't said where in Thailand you are looking, so I have assumed Phuket and here is a link that might help,
http://www.phuketdir.com/category.htm
It would also be worth looking at some of the guesthouses to see if they will rent you a room or apartment for 1,2,3 months.
For example, Paddy's Inn is fairly close to Patong Beach and has the following rates for a minimum 3months stay...
Room with Fan 6,000baht/month
Hotel Room 15,000baht/month
2 bed apartment (1 storey) 14,000baht/month
2 bed suite 16,000baht/month
2 bed apartment (2 storey) 19,000baht/month
Good luck
Diz