We found ours using
http://www.car-part.com/ which indexes the stock of a large number of auto-wreckers. There are a few different databases like that that I found hunting around online.
Our axle came at 800$ fully loaded (calipers, rotors, diff) locally in SLC, UT with 65k miles. I did find that searching for a number of different compatible years/vans would show up more results, even though they are effectively the same axle, so some experimenting was in order.
To swap the axle, for at least the SMB 4x4 conversion we have, the axle shock mounts will need to move, and the angle-shims on the spring perches will need to come over to set proper pinion angle. We also upgraded to 1.5" axle-shafts allowing us to move over the differential from our old semi-float axle.
Brake (running and parking) components were all the same between the full-float and semi-float, so that changeover was easy.
-e