Thermometer Question

"I recently just acquired a BB-Q smoker and it does not have a thomomater in the smoker itself. It's a half barrel design with the smoke pit off to the side. I know that most of the time when you are cooking on the smoker you want to keep the temp at 200 to 220. I am looking at addeding a thomomater and have no idea what kind I should be looking for. Also do I need to have one on either side of the grilling area to make sure the temp is good at either end."