When I was recently checking the UN-SALB (Second Administrative Level Boundaries) project whether there had been any updates for Thailand yet, I stumbled on the Wikipedia article on the related GAUL (Global Administrative Unit Layers) project by the FAO. And finally the weblinks in that article include on to the website of a project named Global Administrative Areas (GADM), unlike the first two a small independent group without the backing of a large international organization.
On that website one finds the boundaries from the subdivisions of most countries in the world, in case of Thailand down till Tambon level. They can be downloaded for free in various formats and are free for academic and non-commercial use, for those who have no real GIS software the Google Earth KMZ format is probably the only which can be used. I had a short look into the Tambon file, with 18 MB size and over 7000 shapes in it Google Earth became very sluggish, and sadly without any hierarchy in the KMZ its very hard to navigate.
So the few areas I managed to check in detail, these boundaries aren't better then those displayed in Google Earth already, both having parts where they must be in contradiction with the actual boundary. Sadly the website does not state any source for the data, or any comment on the accuracy. Thus sadly these boundaries are still not help on my search for authoritative maps.