ev-cpo but increasingly tesla-info.com in my experience
Hank disagrees with every other site but try them for yourself - just having a quick squint at the numbers tesla-info have about 980 US CPO cars, EV-CPO about 920, for the sake of argument they have the same cars. tesla info claim to include some of the hidden stick that Hank says probably don't exist but that's probably more applicable to the new inventory, in a lot of countries including the US, tesla info also brings in a lot of used cars not at Tesla so you can compare the wider market in one place, and then you can choose for yourself which front end prefer between the sites.