Fin, I'm looking at your wheels in the magic book, and I show 2 center caps available for it, so it'* one of these:
9593883 (for RPO N93, PF5, or N89) I'm betting these are what you have, and they'll run about 5 bucks if I guess right.
The other is:
9592965 (for RPO N94) I'm betting these are what you're after, since they'll run about 20 bucks apiece (guessing again....calculated guess though, without running the numbers myself). Since you don't have an SSEi with thes originally installed, the RPO'* won't help much, but maybe an 00+ member can chime in here and say what they have with respect to their RPO code for wheels.
To clarify, these are the EXACT wheels you have, and these are the two center caps available for them.
bud. No other way!