Welcome Dizzy,
I bought a used phone at one of the top floors in MBK while in BKK for about $30 (absolutely no frills) with charger. I've used DTAC service for years with many phones with no problems but there are other services. You can also buy an unlocked phone on ebay fairly easily, just make sure you either get a quad band phone or at least one with the 900 and 1800 bands. I used a tri-band phone (US model) once that did not have the 900 band and the service was not consistent (hence the purchase of the phone at MBK). Also, many years ago one of the little shops unlocked my locked phone for me but it took a very helpful clerk to negotiate the task with the service tech.
