year one, latemodel restoration supply( www.50resto.com), and Mustang Parts Specialties ( www.stangparts.com) are three that come to mind off the top of my head. Tons of others out there.
I'm converting to a black/black interior (who isn't? ) from a black/gray, and I've gotten sill plates, center console, and carpet so far, and I've seen kick panels, door panels, seats, entire dashes and more on eBay for less than Mustang Parts Specialties (which is itself much cheaper than dealership charges). Most of these parts are OEM take-offs from parted cars or wrecks.