If you want one for the outside of the smoker you can pick up a Tel-Tru thermometer. They are solid thermometers that hold calibration really well.
For checking temperatures for meats inside the smoker you can get something like a Thermapen, or a Maverick PT100 instant reads. There are some others that take temperatures slower also but those 2 are the bee's knees.
If you are looking for a single solution for keeping temps of the smoker as well as large cuts of meat. The Maverick ET-732 is a great option. It has the grate probe for smoker temps and a meat probe for meats internal temperatures. It also has a wireless receiver so you do not have to constantly walk outside to check temperatures. It also has hi/lo bbq alarm warning as well as food temp warnings so that it will beep at you should it cross the thresholds. I love this feature for overnight cookers. I can set the hi/low threshold, and food target temperature and when it warns me, I wake up and tend to the smoker.
*Weber Mini WSM* *Weber Touch and Go Performer X3* *Sublime Smoke UDS* *Maverick ET-732 * World's Fastest Blue Thermapen **Pitmaster IQ110*