Before you arrive: You can easily get an eSIM online before travelling and activate it as soon as you land. It’s a great way to skip the lines at the store- some recommendations are:  

• https://www.ais.th/esim-traveller/#/,
• https://www.dtac.co.th/en/new-number/esim or
• https://esimthailand.com

Upon arrival: If you prefer a physical SIM (or even an eSIM), and you’re landing at Suvarnabhumi Airport (BKK), you can grab one from any telecom provider right by the baggage claim or for those landing at DMK, there are counters between the International Arrival Halls 3-6.

We are signed up with True mobile, but there are other providers such as AIS and DTAC, but we recommend True.

For others, international roaming should work smoothly, too, as long as you get your packages sorted in advance!