You may find what you're looking for at Trekwood.com. Here is one wood furnace return air vent cover, size 13.75" x 14.5".
https://www.trekwood.com/products/40...2-Ash-Kona-Koa
I'd guess that's not the correct size, but is a "lead to finding one". I'd call Trekwood at 866.880.8735 to see if they can even get one. Keep in mind, the advertised cost is over $130 and that's before shipping charges are added, so it won't be "cheap"...
You might consider buying a "stamped steel return air vent" from Lowes or Home Depot, painting it to match your interior and installing that. It would be significantly less expensive.