They're still pretty expensive, but PowerHouse has the aluminum ones in stock for $735 at
www.powerhouse411.com . The Shelby GT 500 has an aluminum driveshaft, but I don't know who supplies them or whether FRPP will be offering them soon.
I have the PowerHouse driveshaft and it's a nice piece. It dropped right in with no problems, no vibration, and maybe made the GT a little more responsive. Hard to tell since I've never been on a dyno.