I bought a used wheel and tire off some guy selling them in my local Craigslist. He services fleets of FedEx ground trucks so he had plenty to choose from. He is in Fullerton, CA. Not sure if you want to go that far in getting one.
You can ask around to see where the Uhaul, Penske, FedEx trucks are serviced or even put out of commission and see if you can buy a used wheel. I believe they're all the same as the E450, but check the code on the rim to make sure it matches.