As far as I know Johnny Langton is the closest thing to a company to get them from. He makes them off and on when he has the time to do it I think. If you want something else, you'll have to find a chassis shop that will custom fab some I think.
Former owner of 94 Bird with DOHC 4.6L swapped from a mark VIII
2011 CTS-V Wagon
1968 Torino was an original 289, C4, fully documented numbers matching car. Now: DOHC 4.6L/T-56 swapped, Cobra intake, IMRC delete, BBK long tubes and H pipe. Dynotech Aluminum Driveshaft, 9" rear with TrueTrac and 4.11's.