Have you called any of these metal fabricators and asked them where you could source it?

https://www.chemetal.com/design-categories/brass/

Some of these are metal Laminates. Maybe you could order the sample pack and see if any of them would work.

Hello. We can do this for you. I doubt there are other companies that can do this in the U.S. You can reach us at rachiele.com

https://www.wilsonart.com/antique-brass-y0379-7ca6e21252345fa250ae192918b0e312

https://www.wilsonart.com/antique-brass-y0379-7ca6e21252345fa250ae192918b0e312

If you select brass that has clear laminate over it you won't have to worry about discoloration.

In my previous house I had a custom hood made by Modern-Aire. I'm sure they could make a backsplash piece. I used a antique brass finish on that hood.

http://modernaire.com/hoods/

You do realize that brass tarnishes, and rather quickly. Be sure that you want what it will look like next year at this time. And in 5 years too.

We have a durable topcoat that will withstand fairly high temperatures. Raw brass will turn a greenish color quickly if not clear coated. This is a link to a video that shows the durability.

Hi! I'm sorry to be weighing in so late on this, but we're in the process of installing a raw brass backsplash (I'm excited about the varied patina it will have). I sourced the metal from Metals Depot. They sell sheets of raw brass (and other metals) cut to size. Hope your project went well!

https://www.metalsdepot.com

You probably should read all the complaint threads about brass burners ”discoloring” at the very first use. No clearcoat will stand up to a 22K BTU burner throwing it’s heat at the wall because the pan is a bit wrong sized.

You know, Martha has minions for this. Well paid minions. Do you?