This is the one I got that didn’t take too long to get a temp reading!
https://www.amazon.com/Body-Basal-Thermometer-BBT-113Ai-1-Sensitive/dp/B07KB2QFDY/ref=mp_s_a_1_3?keywords=basal+body+thermometer&qid=1561631721&s=gateway&sprefix=basal+bod&sr=8-3