CMN, I spent hours and hours trying to do exactly what you're trying to do. There is a sticky on ajarn about it:
Subtitles! - Ajarn Forum - Living and Teaching In Thailand
There is a site for Thai subtitles:
:: ThaiSubtitle.com :: ศูนย์รวมซับไตเติลสำหรับคนไทย, Translate Movie Subtitles English into Thai
You can only download the subtitles that are 100%. I tried to do this for Lost Season 4, because the g/f wanted to watch it. I followed the instructions written in the sticky, but couldn't make it work. I even downloaded several different media players. The basic problem is that none of the players recognized the Thai text. You need to download the font as well, but mine kept coming up in some gibberish. After about 2 weeks of trying, I finally gave up.
If you get it to work somehow, let me know and I'll green you for life.