Does anyone know where I could find a Inexpensive CO2 controller/monitor

Hello everyone
l have a propane burner and am looking for a reasonable monitor/controller preferably digital. Never used CO2 before so any advice is much appreciated
thanks

cheapest digital i found was the cap PPM-3
http://www.amazon.com/C-A-P-CAP-PPM-3-CO2-Controller/dp/B000QFMZES/ref=sr_1_1?ie=UTF8&qid=1343716142&sr=8-1&keywords=ppm-3

just be aware that it may not work with digital ballasts near. (believe it was ~20ft)

i have a ppm3 that is 3' from 2 digital ballasts and works great. for the money it is the best.

Nice. Thanks for the info.