Sorry for the delay. Didn't see this until just now.
We purchased this one (actually, we have three of them):
https://www.amazon.com/gp/product/B07F7Z2HZM/
When my wife ended up needing a visit to the ER when she had COVID in December 2020, we checked the readings from the above device with the one used by the hospital and they were consistent with each other (reading in the low 90s). We have also periodically checked consistency between the multiple ones we have, and we have found them generally consistent with each other. It's not uncommon to get readings that differ by 5% or more a few minutes apart, though.
With that said, when we were in Winter Park, Colorado this past June, my 10yo daughter was not feeling well, presumably from altitude sickness. Her SpO2 levels were reading really low (in the low 80s, maybe even a reading or two in the upper 70s, which is potentially serious). My wife and I were driving her to the nearest hospital at 2am. We kept checking the readings every few minutes. Then, all of a sudden, the levels returned to a 'reasonable' range (e.g. low 90s). (Everyone's SpO2 levels were several % points lower than normal at 9,000 feet.)
I honestly don't know if those were errant readings or something else was going on. I also don't remember if we were relying on only one pulse-oximeter or if we were getting low readings from 2 different meters.
All that to say that the best approach, imho, is to get a couple of them and to routinely check your levels when you're not sick, so that you get a feel for what's 'normal' and then you can check for deviations whenever you do get sick.