Well, Rockwell is a well known brand but 1996 was a long time ago and maybe the parts are obsolete. On the other hand, there are still lots of old truck & coach chassis in use and usually there is some parts source for them. Your shop may have simply called their usual distributor and not looked much further. Plus, if they just asked by coach brand & year, they aren't likley to get much response because RVs are often not in the parts catalogs. Rockwell axle model number would get them closer, but knowing the actual shoe size dimensions is the best way to find them. And maybe the part number & brand of whatever is not on there, so they could be cross-referenced to another brand.
Lots of places to hunt, even Ebay. Brakewarehouse.com has a selection too.
I think Rockwell is part of Meritor these days, so maybe a Meritor brand shoe is available.
