Monoprice 4 in 1 sensor
ZooZ 4 in 1 sensor
You can find links to purchase them on those topics as well, both are about $30-$40.
Beware though that both has relative short battery lives, about 2-6 months depending on the settings and amount of use. They run on 2 AAA batteries but @TheSmartestHouse has one in the works that will use CR123 batteries.
You can use AAA lithium batteries as well if you want a longer battery life, alkaline is fine but avoid rechargeable as their battery voltage is too lower to operate these devices.
Alternatively if you have the Z-Wave CT-xxx thermostats (from GoControl, Linear or RTC), they also report Humidity in 3%, 5% and 10% thresholds. The 4 in 1 sensors are also reasonably accurate.