I know that spokes, nipples, cones and complete axle assemblies were supposed to be available, and often weren't, but I never saw rims listed as spares.
Shimano rims do have a part number for ordering spares, but I doubt whether anybody has gone through with the order after seeing the price relative to a complete wheel. Maybe on the Ti freehub DuraAce models it would be viable since the hubs are so expensive.
I know that spokes, nipples, cones and complete axle assemblies were supposed to be available, and often weren't, but I never saw rims listed as spares.