In Indiana there’s a lot of 100yr old barns guys will salvage and use the wood which usually has been exposed, sometimes some old paint etc. lots of times they used wood from the farm so it’s Oak, Ash, Maple,Walnut, whatever was close and they needed removed. Most of it is 1x8”.
Another thing is pallets, I used them on. Wall in my wood shop. It’s generally only 1/2-3/4” but works nicely for trim. It also has some of the nail heads left.
Don’t know if this is a little secret but before you install paint the background black of you could use roof felt depending on location. We also varied the thickness a little on the wall.
On the lower part of the wall I used galvanized sheets(I didn’t but have seen ppp use the same barn roofs so the metal is aged/somewhat rustic). I also used black Walnut to make wainscoting panels around the galvanized.
The flooring I used Cypress with a few pieces of Walnut inlaid and I used square head nails from house of antiques(make sure to run a/grain to avoid splitting, pre-drill) but it definitely makes for a rustic look.
BTW, since everything doesn’t line up(rustic) I’m a D-fine “rustic” carpenter lol. Good luck