Contact Track of the Wolf, and Dixie Gun Works about the horns. If they don't have any on hand for sale, ask for a referral to a hornmaker. These are not difficult to build, and many shooter prefer to build their own so that their equipment has a personalized touch to it. It does take a few tools, and some time. I just saw a couple of flat horns on Dixie's on-line catalog for less than $70.00. I don't care for either of them, personally, but they are well made, from what you can see. Keep your eyes open when you attend major shooting events,( Friendship) and rendezvous, as you can sometimes find what you like at bargain prices. Also, hunt the antique stores, and farm sales, where any muzzle loading guns may be sold. I have found some old horns that were available for very cheap prices. I currently own a horn that dates back to the early 1800's, which had a piece of the spout broken out and lost long ago. I had a custom hornmaker repair the horn because of its age, and I use it now.
As to flattened horns, I have the raw materials to make a couple of these. I expect to make a few mistakes, so about the third one I make I will have 90% of the tricks learned, and will produce a horn I want to keep for myself.