Which hydrometer type should I use? Best brand?
Some people use hygrometers for reptile/amphibian enclosures, you can get these at a pet store. I used this thermometer/hygrometer combo, but I'm not sure how accurate the hygrometer part is (the thermometer part was pretty accurate): http://www.walmart.com/ip/AcuRite-Digital-Humidity-and-Temperature-Monitor/16888914
I bought the Acurite from Wally World like stated above and it's pretty darn good for the price. So far, it has been very accurate checked against a Brinsea Spot check. Mine has been at 99 and 45% humidity for a few days now.
I found that my bater is off by about 10%