Pine boards from solid pallets if you dont mind picking up the pallets than stripping/pulling them clean. Or purchase rough pine/hemlock or whatever is harvested and milled near you that isn't very expensive.
I have shiplap 6"wide pine boards in my 19x30 garage that I exposed the smoth side out. Have had them up for over 10years with no problems keeping as clean as a garage can be kept in farm country.
I installed it a section at a time when I had the money to buy the lumber. Took a few months to do but is well worth it. I can hang just about anywhere I want without having to make sure I'm on a stud as I would if I used drywall nor do I have to be concerned about putting dings/holes in the drywall.
For the ceiling I have sprayed roughsand finish.

Did you install the boards vertical or horzontal?

I installed the two near the ceiling horizontaly the remainder verticly.
Reason was didn't have as much waste as my walls were over 8feet but less than tenfeet so I either added the length I need to an 8footer which ment cutting short pieces or cut some off a ten footer than I would have to be carefull when installing it not to hit the ceiling.
I figured out that installing two of them horizontaly the rest verticaly I wouldn't have to play around with the staggering of seams and having to buy longer boards to make this staggering easier.
This way here I was able to purchase one length and have very little waste.

Sorry for the late reply but haven't been on the forum.