I just stumbled upon existence of this reference book by chance while cruising on the web. Here.
Ordering by mail is possible. Even if you can't read Thai texts, you can read still read the phone numbers to make an inquiry from that Thai web page.
The Thai - Tai-Yai Dictionary has been published since 2010 (B.E. 2552), commemorating the 600 anniversary of King Tilokarach (of Lanna Kingdom, whose capital was Chiang Mai) and 85 years of Rajabhat University at Chiang Mai. Compiled by many experts, it took over 10 years to finish it, and published by RUCM's Language Institute. Price for the book is 500 Baht.
I just noted down that I am interested in buying a copy. Perhaps next time, when I travel to Chiang Mai visit near Mae Rim, I 'll stop by at Rajabhat University at Chiang Mai to buy it and 'save' 100 Baht of mailing cost.
(Now I am rethinking since I am not sure if that will really 'save' money or 'spend' extra money on the fuel driving that far from my new northern home.)