It is easiest to buy a Thailand sim card at the airport once you arrive in Thailand. There are three major carriers in Thailand that offer what is called a traveler or tourist sim card they are AIS, dtac and True Move. I suggest you pick a plan with plenty of mobile data as you will likely need it. The quality of Wi-Fi at a lot of Thai hotels is very hit or miss. Who wants to wait to upload their vacation photos to Instagram until after they get home?
You can find these Thai sim cards at the Suvarnabhumi airport in Bangkok upon arrival. As you enter the arrival terminal, just after you pass through customs, all of the major Thai carriers have booths set up. For those bypassing Bangkok and flying straight into Phuket, you will find a similar set-up there.
The cards themselves are generally inexpensive as the real cost is the pre-paid plan you are buying. If you follow one or more the links above, you can see the duration and the cost of the plans vary. Most plans are geared towards trips of 5 to 30 days in duration.
Can I get a better deal on a sim card outside the airport?
You can get a slightly better deal on a non-tourist sim card if you are willing to pay for a month of service. However, there is a downside. Thailand requires all non-Thai citizens buying a sim card to have their picture taken and a copy of their passport made. These are then submitted to the Thai government. Employees at the airport booths are professionals that do this thousands of times a day. The busy clerk at the convenience store is not.
If the clerk fails to submit the proper documentation, you may find that your sim card has been turned off. It rarely makes sense for someone staying in Thailand for less than 30 days to go with a non-tourist sim card purchased at the airport. The cost difference is nominal. Most people find what they need based upon their usage and duration of their trip for somewhere between $10 to $35 at the airport booths.
What If I Need To Add Time Or Data To My Plan
You can top off your plan at various convenience stores, retail stores and mobile stores located throughout the country. Some of the carriers allow you to top off your plan online. For example, AIS allows you to top off your plan online with a credit or debit card.