I got mine at
www.hubcapkingdom.com
they were really helpful, too. I originally ordered the metal ones, but one of them came off after like one week, so they comped me a set of the plastic ones, and they have always stayed on. They told me that the steelies on xBs have problems keeping the metal ones on, so I would recommend the plastic.
NAPA also has cool little metal caps for the center that only cost $2.13 apiece.