I had a bit of trouble tracking one down myself. Oddly enough out of the 5 petstores i visited, none had any nor really even heard about them. Would certaintly explain the poor chameleon care in all the shops. I eventually visited a store strictly for plants- it was a hydrophonic grown exotic plant shop. They had about 4 different kinds, ranging from 15-60 bucks. I went with the midrange that includes ambient (roomtemp), humidity, temp at the target spot (where you put the wire thingy)
My gauge also keeps track of history. I reset it every morning as i go to work, and when i come back, a few clicks of a button will tell me the high/low of all 3 - humidty, ambient, and measured spot.
I also looked at home depot and didnt have much luck there either, you luck may vary tho!