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I got one from AutoVentshade. Their large generic floor liner is 48"x48"x2" deep, which fits perfectly with the 3rd seats folded. Plus, it was only $26, instead of $75-100. The box said black, but
When the seats are up, I found a Rubber Queen mat that is 48"x14" and fits pretty good behind the third seat. It also fits good as a floor mat in front of the third seat. I like the deep grooves for catching all the mud that the kids drag in. In the cargo area, it catches spills, and the texture keeps things from sliding around.
If you could get a liner that was kind of "floppy" (instead of the molded, rigid plastic kind), you could probably rig some sort of way to connect it to the rear seats, so you wouldn't have to shuffle it around when folding/unfolding the seats.