If you'd like, you can PM me you VIN and I'll enter it in FAST (the subaru Parts catalogue software I have installed on my PC). It gives me Part#s specific to a VIN.
Or do the shadow chrome wheels on PostFL SpecBs have the same colour centre cap? If I was at home I'd check mine. Or are you sure yours are not the Shadow Chrome 18" wheels? Same shape but darker silver. I thought all SpecBs were the shadow chrome, but maybe that's just just postFacelift.
You can use online Part catalogues like
http://www.opposedforces.com but it's not VIN specific. So you'll have to check if it says whether it's for a shadow chrome wheel or not.
EDIT: Opposed Forces has been useless - it shows about 5 different centre cap styles but no description or Part#s.