Might try here: Tom's Differentials 15709 Paramount Blvd Paramount, CA 90723-4332 Phone: (562) 634-8431 I have never bought anything from them, but I know they have been around a long time.
Pete, if you want new, get it from Currie, Ford don't make 'em any more. Closer to home for a new one (Currie stuff also), call Sydney Compe***ion Warehouse 02 9891 5088. Drum to drum 28 spline about $2300 - a bit more for Moser 31 spline axles, your gear choice of course. Tell James Tony sent you.
Not sure if these guys do a complete axle but I have just got a complete 9" 3rd member from them at a very compe***ive price. Might be worth a try Valley Springs Custom Differentials Ask for Clayton and tell him Paul from the UK sent you 001 209 810 7326 Thanks to Limey for putting me onto him. P.