I'd like to put a rear bumper on my '88 SR5 4x4. I doubt my chances of finding a functionally identical donor *with* the bumper are good. Are there alternative donor vehicles?

 

Have you looked at the 4Runner's?
If you can't find one from a wrecker/pick and pull.
Here's your options:
Leave it.
Make one.
Buy one from one of the many awesome vendors (Wab-fab, Marlin, Trail Gear).

 

Maybe someone here is about to upgrade to a tube bumper and is willing to sell you there stock one to help offset the cost.

 

You're not likely to find a working bumper on a salvage vehicle (why do you think it got salvaged?). But salvage yards know that, and they often distribute aftermarket parts for items commonly busted (mirrors, bumpers, etc.) I paid $145 for a front bumper AND valence for a '94, and I'm quite happy with it.

Something to compare to when you're thinking about driving to the next town to look at a wrecked vehicle.

 

I see a lot of vehicles at the junkyard that were either donated or drove in. Lots of clean vehicles (rusty but not wrecked). 4runners are way more common that the pickups out here.