There is a foreign affairs department in Chiang Mai.
Looks like a new office.
It is in that huge exhibition area on highway 121 on the left if you are going clockwise.
In about the same area as the sports stadium.
Pardon me for the vague directions but if you need them I'm sure you will find your way and be so happy that you don't need to hang around in Bangkok just to get a document stamped.