Hit the hitchen gadget section of wally world or target and pick one up. You can get a meat thermometer with the needle and dial or an inexpensive digital model that will be a little more accurate. I used the former for a few batches and it worked fine until I got a digital one.
These are cheap and worked for me at first:
Then I got one kinda like this:
Some folks on here go even fancier. Just depends oh how much you want to spend. A simple one will only run you a few bucks. My digital was around 10-15 bucks I think.