You can find frame parts at out of doors mart here:
http://odmrv.com/catalog/index.php?m...ort=20a&page=2
If you're taking your frame to a welder, I'd suggest just having them fabricate the pieces you need. An outrigger is just a piece of steel channel with a curve cut into it. If you don't need the holes in the frame members, you can use regular steel stock and save some money.
N.B., the current generation of Airstreams are a few inches wider than they were in the 70s.