In July I got an AIS sim, paid 200 bt for unlimited data for 30 days and it was fine for online browsing. I think they call it a tourist sim. I used some online website to put 50 bt on the sim for calls which I never used. In September I went back to the AIS shop in Silom Complex, paid another 200 bt and it worked well again for my 28 days there. Good value.