If you don't have any luck, Lowes and Home Depot usually sell plastic Captain Wheels for their kid's outdoor play sets.
You could "rotten it up" yourself with a little paint and cheesecloth.
I've seen them made with the wood wagon (stagecoach) wheels that Big Lots, Harbor Freight and some other stores sell. Just attach some decorative posts around the edges, and possibly file the wheel rounder if you want.