To all of the above, I would add, almost any small, local museum in every state in this country. An example of this, that comes to mind, is The Plainsman Museum in Aurora, NB. They have a number of rifles that played roles in the history of that state. Ditto, the others.
Almost any large city, or county museum, seems to have a few or more, which early settlers used in that area. Historical Society museums are a good place to look, too.
The Lebanon County Historical Society, in Lebanon, PA has examples of Beck and Beyer rifles as well as some other pieces. Of course, the Historical Society of PA, in Harrisburg, has rifles including the finest Issac Haines gun in existence. You just have to have the time and the desire to look.
Regards-Dick