Craigslist, Facebook Marketplace, OfferUp, eBay, Gamestop
I would try CL, FB, and OU first, as usually people are just trying to unload stuff, might have been sitting in a closet for awhile, might not be updated. You can reach out and tell them how to check the firmware.
eBay would be the next choice, and probably the easiest place to find one, the issue being that people are more likely to know how much lower FW units are worth.
Gamestop would be my last choice, you are going to pay more, and technically they are supposed to update the units to the latest version. Now in practice, that isn't super likely, and you are still able to ask if they can turn it on and check the firmware.