If you want to use your own mobile phone and have a local SIM card for lower the telecommunication cost in your international trip, please check your phone for it is locked or unlocked before this trip.
An easy way to tell what status your phone could be is the money you pay for it. If you paid $, $99 or $199 for a top model Apple, Samsung or Nokia phone, a price that low means your phone has been subsidized, which is locked. If your phone is locked, you won’t be able to use a foreign SIM card. If you paid $500 and up for a top model, your phone is unlocked and you are free to use SIM card of any region.
7-11s, airports and online shopping at Amazon, Ebay are where you can easily compare Thailand SIM cards and buy one. If you want internet, you may also buy a pocket wifi or mifi device which are accessible online.