As Stuart says... But also be careful at the airports, I've heard of several occasions people buying what they thought were longterm deals and they only lasted a few a days in reality.
Get a sim and top up, install the app and browse the deals... Take a deal shorter or less than need, you will be bombarded with offers to extend or improve your deal....
There seems to be doubt on year long deals/sims... Yes there many deals that have a full year and they are great deals. I got one for my son 1350baht, 30mbs unlimited and free calls to dtac. Many similar on lazada etc...
In this case I suspect it may have been a bit of a scam, I've heard of these deals sold in shops as 12 months and only are infact only a short term deal, some as short a week! This was happening at the subvarnibumi phone shops especially due to the customer being long gone.... 😉