After buying several different types of specialized art board and various thicknesses of card, I tried out the backing from a legal pad on the recommendation of someone here at BGG.
It's the perfect thickness and cuts more easily than the more expensive art boards.
So, find a legal pad or similar pad of paper (letter sized, or what-have-you), rip off the cardboard backing and use that. Cheap, easy to find and easy to work with. Wish I had found threads telling me about it prior to spending $20+ on testing various other types of cardboard.Caveat
- thickness and quality of legal pad backings vary - check the thickness and quality against your Combat Commander counters before
sticking your expensively printed labels to them! Also, some people prefer a very light-coloured counter when looking at the counters from the side (similar to the stock that GMT uses) - using most legal pads will not achieve the same "look" - you may or may not care about that (I've learned to never underestimate the picky-ness of wargamers!
What Cindy (poster above) calls "chipboard" (hadn't heard it called this before, but seems to be the correct term) is the same thing, apparently.
If you don't have ready access to legal/letter pads with that type of backing, you can also buy it at Walmart and Staples:http://www.staples.com/8-1-2-inch-x-11-inch-Staples-Heavy-Du...http://www.walmart.com/ip/Medium-Weight-Chipboard-Sheets-12X...
(note to Canadian readers - these are US links - not sure where you go in Canadian locations - no hits found when searching for "chipboard" in Canadianized versions of the same sites)
Also note that if you are printing PnP counters onto full sheet labels with ink-jet printers at home, consider applying one or two clear coats of glossy (or matte - your preference) art fixative spray. It will help reduce the amount of smudging and smearing that may occur if you are going to be handling the counters frequently.
Laser printed output typically has a fairly durable fusing, but some people fix that as well. It will work whatever you do, these are just recommendations for added longevity.