You can find them off of a focus as mentioned above but it was primarily a secondary option wheel for the SVT model:
2002-2004 Focus SVT
2009-2011 Focus SES
Most that I have found in craigslist searches are not terribly expensive, but usually not in the greatest shape.
A real find is the Saleen Ford Focus S121 N2O. It's not the prettiest 4x108 wheel, but pretty rare to find.