Meh I just got one from walmart for all of 5$, but nothing beats an actual thermostat since that will keep it at the temperatures you want.

And if you have something testing the temps of the hides you shouldn't need another thermometer. However center is probably the best for it. And an inch above the substrate will give you the temperature the snake is experiencing the most. While if you put it up higher you will be getting the temperature of the air in the tank. So it's really up to you. Find out the temperature of the air the snake is exposed to, or the temperature that the snake is getting from bellow it.