I have an Oregon Scientific weather station so I use the OS temperature and humidity sensors. The sensors can be used without the main station and they are very accurate. Here are a couple of different models:
THC268 and
THGR122NX.
They are a little on the expensive side (but under $40), but if you have or ever get the station you can log the information to watch how your humidor fluctuates throughout the day.
BTW, when's the next time you are going to be in PDX for a Herf?