Steve Wall Lumber in Mayodan, NC is where I've bought before. You can mail order if you don't want a little road trip.
Do keep in mind it's "Spanish Cedar" and not the "Cedar" you see sold for closets. Two totally different things and will ruin cigars if you use the wrong one for a humidor or to line a coolerdor.
Also if you cut/sand/grind on it make certain to use a good mask and filters along with some decent eye protection. SC dust from working it is pretty nasty stuff.