Ebay, people often sell their OEM radio left over after replacing it with an aftermarket radio, usually pretty darn cheap compared to the price new and they are usually gently used.
Dealer, just order a new one, it will be frustratingly expensive.
Dealer with online sales, same as any other dealer, just has an online sales department selling the Dealer parts at a discount.
Internet searches, perhaps a company is selling new OEM radios or used/repaired OEM radios.
Finally go aftermarket and get a new Head Unit that includes NAV, most including NAV are the more expensive models and are fully loaded with features, including back-up cameras and auxiliary output for rear screens, etc...
The issue would be can the aftermarket stereo connect to the existing back-up camera and rear screen TV. My aftermarket head unit, with an optional XM radio unit connected, the OEM Satellite antenna doesn't fit, but you can make it fit by taping it down and the OEM antenna gets the signal just as strong as the included antenna that your suppose to use.
Go to Crutchfield: Car Stereo, Speakers, Home Theater, Pro Audio, 4k TV
they have lots of resources to figuring this out.
And another option, since you have to pay so much for the NAV, just get the control head without the NAV and use your phone. Although a couple years ago when I upgraded to an aftermarket stereo, the connection between head unit and phone was still rather immature. I could use my phone NAV on the Head Unit Screen, but I couldn't listen to the radio or recorded music while I did it, as well I had to connect two huge cables to the phone. Perhaps its more worked out now, and you can actually use your phone NAV on the screen while still listening to the radio and NOT use ungainly cables while doing it. i.e. Android Car Talk, or something like that.