My gauges are the digital ones from Wal-Mart... they are under $10.00. Just make sure you don't place them where they can be knocked in to the water (I learned that the hard way, LOL). You must calibrate it, though. Both of mine are off, which isn't a big deal to me, because I just add/subtract the amount that they're off by.
In the next aisle over, they have those hooks from 3M that you can place on the side of the tank, and then just put the humidity gauge over the hook, and voila!