A basal thermometer is the device used to measure basal body temperature. Basal body temperature is the body temperature upon waking and it is least affected by external environmental factors. A basal thermometer requires an accuracy of at least 0.05 degree Celsius for the purpose of capturing small changes in body temperature. Depending on the manufacturer, a basal thermometer may be a glass thermometer, or it may be a digital thermometer. You can probably find basal thermometers at specialized drug stores or pharmacies.