I purchased a CE marked finger Pulse Oximeter online from a firm called Pulmolink (based in Bristol IIRC) a couple of months ago. At the time they didn't have many left as there seems to have been a run on them due to their helping in covid diagnosis. My "smart" watch also reads SPO2 and the two devices are within a couple of percent of each other. I had had my SPO2 checked in hospital recently and it was OK then, so the DIY devices are probably near enough.