Exactly - I bought my 17" Enzo 5 spokes (see my avatar pic) - with tyres for £300 on Ebay, they were fitted to a Gen7 Celica for 3 months so were 'as new'. Don't get ripped off, the only way we should be ripped off in this country is by Mr T (last resort) and the taxman, you can avoid Mr T but not the taxman!!
Try Ebay or the wanted section on here, people are always getting shot off older alloys so you should be able to pick up some centre caps no bother.
Cheers Rad