It is possible - if you have a good lumber yard where they really know their lumber, and are willing to do the cutting and planing for you (which you will be charged for) or if you have a friend who has a good woodworking shop that can rip and plane the untreated cedar planks for you.
Watch out for the obvious fist choice - cedar shingles. These are almost always treated with a fire retardant chemical these days.
"It ain't what you don't know that gets you in trouble. It's what you know for sure that just ain't so." - Mark Twain