For brooding chicks... I have never needed to use a thermometer. I go by what the chicks are doing. If they are all huddled together in a tight buncle under the light, and chirping loudly they are uncomfortable. If they were not under the light, and huddled at the edges of the brooder chirping very loudly, they were uncomfortable. If they were milling about the brooder chirping softly, and playing they were just fine at whatever temp the brooder was at. However, I did put a thermometer in to see what the temp was a few times. Never once was it at 90 degrees the first week. It was more like 80 degrees, maybe 85 for first week. I'd rotate the light at the end of each week to bring the heat down as they got bigger.
For incubating...You can get thermometers a few different places, but some are accurate, and some aren't.
Walmart sells an acu rite one that is ok for thermometer, but not so good for humidity. There's a honeywell brand you can get also. It's ok too. Another place you can look is at a pet store that sells reptile equipment. They sell thermometer, and humidity guages. Some are made by flukers, and are accurate. For a hygrometer, you can go to a cigar smoke shop. They have some very accurate hygrometers ( both digital and analog) you will be able to callibrate both.
Either way, for incubating, get to or three different thermometers/hygrometers. So you can run an average. Do a test run before you stick any bought eggs in, and you'll get a better idea of what is accurate and what isn't.
Hope that helps.
Bluemoon