Go to your local big box store (Walmart, Kmart, Target, ect.), in the cooking section find the thermometers, look for a probe oven thermometer. It will have a metal probe that is stuck into the meat, then a long metal braid or silicon coated wire that you run out of your smoker to the display unit. The ones at the box stores are usually Taylor Made or something similar and cost $9 to $12 bucks, they work fine, just make sure when you clean the probe and wire you do so by wiping them down with a disinfectant cleaning cloth or a sponge and a good cleaner. DO NOT put them in the dishwasher or submerge the probes in water - water gets into the joint where the probe an wire meet and kills the probe.
If you have a little more $$ to spend I would highly suggest getting a Mavrick Read-Check or Mavrick ET-73. They have a wireless reciever that is really, really nice to have in the house near you. That way you don't have to keep running out to the smoker to check the temps.