Search Reddit, lots of doctors and med students discussing which of the cheap ones work. Nearly all are sold out though. I bought this one last night based on Reddit feedback for $35 but it's already sold out: https://www.amazon.com/gp/product/B01HSAG8BE/ref=ppx_yo_dt_b...
Thanks! A place called Medkioskinc.com apparently still has stock of those (at least they did ten minutes ago when I looked), for anyone reading this thread.
If you've got $300 to spare, this one is allegedly FDA approved: https://www.masimopersonalhealth.com/products/mightysat-fing...