Small ones are pretty cheap
https://www.amazon.com/Hygrometer-Digital-Thermometer-Temperature-Humidity/dp/B013Z8FH6A/ref=pd_sbs_201_4?_encoding=UTF8&pd_rd_i=B013Z8FH6A&pd_rd_r=22Y8E22YPNSV3W8PGRX7&pd_rd_w=QLN1k&pd_rd_wg=e80gD&psc=1&refRID=22Y8E22YPNSV...