I bought a digital Acurite Indoor Humidity Monitor yesterday at Walmart for $7.97, they were next to evaporative cooler parts and supplies and other minor plumbing supplies, it also registers temperature. There were some other thermometers in same spot.
The hygrometers/thermometers I've found in the garden section..There are accurites and springfields..The thermometers with the probes are in the cooking section, they don't have the humidity part. I always use the thermometer/hygrometers since I don't use the water wigglers that need a probe attached. Water wigglers are a cheap kids squishy toy filled with water that can be used with the probe to simulate what the the temp inside the egg should be...
Other hardware stores you can find the Taylor brand but my Accurites and Springfields work just as good for me.. I have a Taylor I don't use...