I don't know about Chiang Mai, but all of those lenses (possibly except the Nikkor) are available in Bangkok. The guys at AV camera (
http://avcamera.com/) are friendly, and might be willing to send by EMS if you pay in beforehand.
Apparently, the Tokina 11-16mm f/2.8 is extremely sharp, even at f/2.8, and much sharper that the 12-24, which is not a bad lens. It's an ideal combination with the Tamron 17-50mm, which is also sharp. If you don't like to change lenses, the solution is called: Two bodies

Yeah, yeah, I know, it's a hassle to carry two bodies, but if you don't want to compromise on quality....
Many people seem to be happy with the Sigma, but it has some strange distortions and a very slow max aperture. I never liked it, but then I don't like slow lenses in general. If God had meant us to use slow lenses, he would have made the D700 much cheaper
