In my experience, hygrometers aren't great, and it's just something you have to make do with.

I've used some of the analog hygrometers (including the one kitedemon linked, purchased from the exact same place). I've never found one that was decent, and the calibration on them always slipped shortly after calibrating them.

There's lab grade hygrometers, but they're pretty expensive. I've never used one so I can't comment on them.

For myself I've settled on a cheap digital unit from the hardware store. Every time I've checked its accuracy against the humidity reported by the weather station, it's always been close enough, frequently enough, that I'm confident that my enlcosure is within the acceptable range, and so far I've seen no indications from the snake that it isn't.