Very specific post, but this blog is all very specific tips and tricks anyway, so why not.
You find yourself in Chiang Mai, and even though you have T-Mobile's excellent roaming capabilities via their new Simple Choice plans (heartily recommend!), you want a local number so local folks can contact you more easily.
If you're south of the city near Hang Dong, go to the Big C at intersection of 108 and 3029 (Just at Southern tip of airport). I suppose if you're Central or North, any Big C would do?
You want AIS because they are the best according to local advice. I hear DTAC can also work, but AIS has good English language skills and best network. They appear to be branded as "Telewiz" for mobile stuff though mostly.
So Find the "Telewiz" kiosk. Lime-green on white swoopy color branding. It's on 1st floor in the mobile phone grotto.
The new SIM activation lady had no line and was positioned so she'd work the outside of the booth, right over the little envelopes of new SIM cards (marketed as "FREEDOM SIM" or "FACEBOOK SIM" etc). There were lines with number tickets for regular service, don't wait there and waste your time like I did.
If you're just passing through you'll want a post-paid plan, the FREEDOM SIM. Was 280THB (8USD) for me to get the SIM, 3GB of Data as a sign-up bonus with 1GB monthly and a small-ish preload of time plus the SIM activation etc. Not bad at all.
Lady set up all the necessary phone settings (APNs etc)