Register Grilles

We moved into a house built in the 1940s. The register and grilles return must be replaced because they are very dirty and were painted, probably with lead paint. I measured the size of the opening 5 "x10". I can not find this size on the net. Suggestions?

Maybe they used a 4X10 and just had a 1 / 2 "gap covered by the edges of the register. Vintagehardware.com or Try heatregisters.com

Odd. It is a fairly standard size. Have you tried eBay?