I would recommend that you consider, or at least take a look at, the powder horns from
October Country in the state of Idaho, USA. Their powder horns are simple, rugged, airtight, traditionally styled, reasonably priced, and they give you a choice of right or left side carry.
I have made a few simple powder horns, and I have bought a couple. I bought one of the October Country Frontier Horns a few years ago, and have no complaints or regrets. This is the one with the octagonal spout, priced at $81.11 USD. this photo is from the October Country Website:
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Since I bought mine, they came out with another horn they call a Yellowstone. This one has a carved ring on the neck:
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They sell the Yellowstone horn for $67.60 USD. I would consider it a "best buy" in the marketplace right now. Either of these horns should satisfy the most discriminating traditionalist, yet they are well suited for practical use. The only thing I would change would be the tip plug, which does not fit very well and does not look "correct," to me. Something whittled out of softwood makes a better and more authentic plug, and is a simple project for anyone with a pocketknife.
Most of the commercial horns you see are clunky, with poorly carved (or uncarved) necks and grossly oversized "acorns" on the butt. The horns you see above are very authentic. Custom horners can make anything you want, but they will be priced much higher.
October Country is probably best known for their leatherwork. I have not bought any of their bags or pouches, so can't comment from personal experience. I've never heard anything bad about them. My only issue with these is that October Country calls them "possible bags," which they aren't, but that's a matter of semantics. Check them out here:
October Country Bags and Pouches
It's true, you get a great deal of satisfaction from making your own gear, but it is also true, as stated above, that not everyone was a craftsman, even back then. Traders carried finished powder horns to the remote frontiers.
Good luck, my friend. I hope you find what you need.
Notchy Bob