A lot of people like these, -> Digital thermostats on Amazon because they're fairly inexpensive. I have several myself and they work fine for most applications. I personally like these -> reptilebasics ve-200 better because they're proportional and I think a little more accurate (but more expensive).
I attach the probe to the heat mat and use a separate thermometer to measure what the surface temperature actually is. (I like these -> reptilebasics TG-1 for spot checking temps.)