Hydrometer or Hygrometer? A hydrometer measures specific gravity/relative density of liquids. A hygrometer measures humidity.
If it is Hygrometer you are looking for, definitely skip the "pet" branded products. My personal opinion of those involves statements about biologically obscene activities involving goats...
I use hygrometers from a Tobacconists. Yes, that is correct, I use cigar humidor hygrometers. I find them to be less expensive and more reliable.
This link shows the brand I use, but I buy them locally:
Cheap Humidors HygroSet Super Accurate Digital Hygrometer