I did a quick search using Google and the search phrase "jews +harp +fort" and got a seemingly endless list of hits that makes me think nearly every frontier fort was well-stocked with the instruments. Here are just a few of the links to articles discussing fort-excavated jew's harps.
You may have already tried this technique, but I wasn't sure because of the way your question was phrased.
Here are links to a few of the articles, one in the newsletter of the JH guild. They'd probably help you if you haven't contacted them yet.
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