You've also got to be careful with the offset if only buying one wheel. Omanyts on the R32 have an offset of ET51 whereas those are around ET44.There are a couple floating around on eBay but the prices they want for the condition they are in is ridiculous.
I have been keeping an eye on the for sale section aswell, but they don't come up often.
Below are a couple of links to 18" omanyts i found on eBay if they are of any use to you.
CAUTION : REPLICA BELOW
Dont worry. Paypal side with the buyer 11/10There's 1 for sale on eBay but I don't like his feedback score
Puts me off