I have a media closet that holds all my AV gear. As of now, I have a fan plugged into a smart plug and set to come in the evening when we are watching TV. However, I would love to just have a tempature sensor so the fan will come on once a certain tempature is met. I would like to get something from Amazon since I have a gift card balance. Will a motion sensor that also reads tempature work in my application. I also see contact sensors that reads temperatures but not sure which sensor will work best for my application. Any advice is greatly appreciated.
Don't be so sure about that. Lithium batteries last a lot longer than alkaline in my experience with sensors (though you could negate that some with rechargeables at the inconvenience of still needing to change them more). With your sensor hidden away as it sounds it might be, you could probably get creative and solder any 3V source to the battery terminals and then not worry about it at all.
Late to the party, but if you want other suggestions, I have a SmartThings motion sensor β the 2018 version β that turns a fan in my living room off and on based on temperature. Works great.
If you want a USB powered sensor that responds quickly, you could try one of @iharyadi's awesome environment sensors. These are zigbee and work great. Not sure if he has any left for sale right now, as he is always tweaking their design to make them better and add more features.