BBT thermometers

MaKayla • 28. Wife. Lakelyn Abigail 7/24/2018 Seanna Morrigan 09/26/2022 😍 Miscarried 6/20/17 & 8/30/21
I would like to start using a bbt thermometer but have NO idea what kind to get. I don't want to spend more than $25 if I don't have to. Any input is appreciated!