I purchsed a 'Happy' sim card on arrival at BKK airport - cost 200 baht. There are several booths for the different service providers - all in a line. They do have different rates so worth spending 5 mins to check it out.

It was fine, top ups at any 7/11 or convenience store for 100 at a time. I used it for texts and it was fine - a little expensive compared to cards such as Hong Kong and China - but easy to use and never any problems with messages going out or coming in and converage was good for the city.

All the ones at the airport will unpack, load the sim and activate for you free of charge.