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.

http://www.jewsharpguild.org/jhgnews26.html#picarch

http://books.google.com/books?id=r03BQK5WdHQC&pg=PA804&lpg=PA804&dq=jews+harp+fort&source=web&ots=ZhfawbQCLG&sig=lZqfd_slKALCkcIv3UNfXtu4XjQ

http://www.rootsweb.com/~nytryon/forthunter.html

http://books.google.com/books?id=4voeuWmbd5MC&pg=PA81&lpg=PA81&dq=jews+harp+fort&source=web&ots=h-JoTKldEg&sig=WzScLcBrfUZ2Vh9BlPIWddt-u9E