I don't mean to be a smartass (well, maybe a little), but if it's boiling, the temperature is 212 minus a degree or two for every 1,000' elevation above sea level. No thermometer required.
However...a means of measuring temperature is quite handy when cooling the wort prior to pitching, and it's REQUIRED if you're going to do any mashing.
For a quick read, I use a cheap digital thermometer that came free with some other brew equipment. It's similar to
this one:
