I'm guessing most SCed VXers want to keep around their old manifolds in case they ever have to remove the SC for repairs or something. If you are looking for an inexpensive one and don't want to order a new one (from St. Charles Isuzu or elsewhere) I'd suggest looking at some of the online used parts sites / junk yards.

Or maybe you can get someone to send you theirs on the promise that you'll send back yours once your project is complete?