I was just recently shopping for old register grates myself. I ended up having to buy new ones at the store because I needed a specific size.
Ebay is the most convenient place to buy register grates. Just do a search and you'll find them, usually around ten dollars.
If you don't like any of those, I know of an antique store in Cincinnati, Greg's Antiques, that sells architectural salvage and other antiques. They have piles of them. The drawback is they cost around $40 or so apiece.
Good luck in your search,