I found this site with a google search. I don't know anything about the company. http://www.mesaperformance.com/web_store/web_store.cgi?page=accescat/valvestemcaps.html&category=yes&cart_id=194165.15776
Scottsdale Ferrari have them in all colors - black, silver, grey. I would imagine all dealers carry them (in the US anyway). Good luck with your search
I bought a set from www.brazilshopping.com. They are the theft resistant design that require a little tool (which is included) to remove from the valve stem. With shipping, it was under $30 for a set of 4.
As always, Ebay has them. Although the Brazil shopping site looks to be better value. http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&ih=008&sspagename=STRK%3AMEWA%3AIT&viewitem=&item=180166558522&rd=1
Here's a store that's a little closer to you. http://www.lapassione.co.uk/?pid=display&cat=0&sub_cat=93 I got a set of these for XMas. IMO they look great on the older cars, but not so much on my F430. The chrome doesn't go with the wheels.