Been searching for the same thing and only found them in Ebay.....for way too much (at least the Campy ones I found at $25 bucks with cables).
Been searching in older Bike shops in the area, but they seem to be all out of it, except for a couple who thought they had one in the shop owner's junk drawers at home...never heard back from those guys about it..... Sure looks cool on a classic bike, specially if you have a nice derailleur to show off. I'll check out the other sources posted on this thread!
Chombi