It's funny that you should post this request yesterday when I was thinking about the registers we had in my childhood home. In those pre hair dryer days, my Mom used to make us hang our wet heads over the register after they were shampooed. How I hated that! But they sure were nice to stand on!
Good luck with your search. You could also try a place that sells old house parts. We have a terrific one here in Maine called Portland Architecture and Salvage.
Reggio Registers helped me get grills for some non functional vents. They are wonderful to work with - gave me ideas to decrease what I had to buy! Give them a call and explain...The same Reggio registers in VanDykes are 2 times the cost.