I am also replacing my register covers, room by room as I paint. Be grateful that yours are a standard size - the one I had to have custom made cost over $100. Old houses with retrofitted hvac is interesting. Need to order 2 for the room I'm currently working on, I think they're standard-ish.
I just get them at lowes, menards or home depot. I prefer the steel. They may cost a bit more, but I expect them to last much longer than other materials. Considering the age of the ones I'm replacing, I expect these to last a very long time. However, mine are all in the wall somewhere, not in the floor. It helps.