You might find some of the iPod adapters on ebay are cheaper, if you don't need the multi-country plug things, eg:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=280084756587

...or for car use this is ok:

http://www.ledshoppe.com/Product/com/CA4003.htm
(all their products include postage in the prices!)

If you have a spare 12v adapter you could make up a cigar lighter adapter socket lead to work with that unit above.