You will have to register your SIM.
These look like the current "Tourist" oriented SIMs:
https://www.dtac.co.th/en/prepaid/pr...-packages.html
(Note the new 6 month/50 baht and 12 month/100 baht expiration options, good deal and a nice offer in case you plan tor eturn.)
http://www.ais.co.th/roaming/visitin...veller-en.aspx
http://truemoveh.truecorp.co.th/3g/s...try/2258?ln=en
Any of these which might meet your requirements should be adequate.
Maybe still ranking DTAC and TrueMove H at the top, followed by AIS, simply because AIS has very little spectrum, and it is shared with ~ 34 million customers.