Try ford-outlet.com. Prices range from $21.66-27.69. They're also good about finding stuff that might not be listed as they basically seem to have access to the whole Ford lineup but at a discount.
Personally I would stay away from those department store hubcaps, they just don't look as good as factory and you know they're cheap when you see them.
I suppose you could also try a junkyard, but how many would you have to hit before you found one?
