I have a 2004 z4 with top Hifi professional version, I have navigation as well, if you have a spare amp you can help me with please shoot me a pm or if your in the states please shoot me a text thanks 732-672-9208
Search Amazon.com. I just bought one for $118 (USD), delivered, from Audiosavings.
There was also someone from Monterey (CA) selling one (used) on Ebay this weekend for $75. It received 0 bids - so it may get reposted.
Looking forward to my installation job after just having spent a couple of hours successfully locating the 3-pin connector in the boot.
Amp might still be available;
http://forums.bimmerforums.com/forum/showthread.php?2014380-Parting-Out-2005-BMW-Z4-Olive-Green-Beige-interior-3-0-53k-Miles