Any digital BBT thermometer should work just fine. If you are charting if Fahrenheit you only need to chart to the tenth of a degree (0.1).If you are charting in Celsius you’ll need a thermometer that measures to the hundredth of a degree (0.01). It is very important if you are charting in Celsius that you have a thermometer that measures to the hundredth degree because if it only measures to the tenth degree it will not recognize your temperature shift.
(P.S. Want a highly accurate basal thermometer that's fast, fits comfortably in your mouth, and doesn't beep? Try Wink, a bluetooth basal thermometer that automatically syncs your temperatures and the time your temperature was taken to the Kindara app.)