Most people use purebites. However, my cats don't like them. My diabetic gets inaba lickable treats during some of her tests, but to be honest I don't know the carb count on them.
I know some people give freeze-dried treats (get the dog bag because it's bigger for less money. You just have to break up the pieces). I tried these and my buddy Lou didn't really care for them (ECID though). I currently give Lou bonito flakes as a treat. I get these from Amazon HERE.