Purchasing a sim card in Thailand is easy. SIM card/phone shops are EVERYWHERE, you can even pick them up at 7/11 stores.
There are 3 major providers to choose from: DTAC (which trades as “Happy”), AIS (which trades as 12Call), and True Move (or Tru).
Prices and plans vary and change often, but for around $3 you get a sim card, usually with $1 free voice calling credit. about 7 years ago