This monitor works great in that the results are immediate, easy to read, and require only battery power. I love the large size of the numerals on the monitor because I can read them without my glasses. I also like that the battery power shuts off a few seconds after use so the batteries should last a long time.
https://www.amazon.com/Santamedical-Oximeter-Fingertip-Saturation-Batteries/dp/B08HH86RT2