I believe the Alpina B6 Grand Coupe has the 20 inch wheels. Check this out
http://www.alpina-configurator.de/serienausstattung.php. I have been checking them out myself and they are quite expensive. Now that they are sold by Bmw dealers they are available through their parts department.
http://realoem.com/bmw/showparts.do?...03&hg=36&fg=15. One thing to note is that the valve stem is in the hub covered by the center cap rather than on the rim! Makes the TPM thing an issue also