It is pretty easy to get a sim card in Thailand. The best sim card is AIS Traveller. The sim card costs 399 Baht and gives unlimited access to the internet for 15 days. Alternatively you can buy the sim card online in advance. You can select, register and pay for the sim card based on your package (Happytourist 299, Happytourist 599 and likewise) and pick it up at the Suvarnabhumi airport.