I found my 2007 Lincoln Navigator L 4 years ago through Copart USA. It's an auction site with auctions around the country.
LKQ Pick-Your-Part also has Navigators every so often, some are in good to excellent condition. LKQ Pick-Your-Part is where a lot of charity auto donations end up. Recently, I was getting that, tick-tick-tick so I replaced the timing components along with the chain tensioners, I put in new rocker rollers and lifters, a new VCT solenoid, a new alternator, a new fuel pump, a new water pump, a new thermostat, a new front seal, a new oil pump, a new oil filter, new front cover, cam cover, and oil pan gaskets, a new fan clutch, new a/c condenser, and a/c seals.
There are many things that I like about the truck but one of my favorites is being able to load full 4x8 sheets of plywood or drywall in when I put the rear seats down. Good luck in your search.