Lion Country Supply in Port Matilda PA, just south of State College

Roger Bain and Monty Whitley have shops in Waynesboro, PA

Cabela's is just north of Harrisburg.

If you're far enough west, Hendershot's in Hancock MD

All have websites to give you an idea of what you might find at each.

Tom Gibbons at Executive Buying Service in Lancaster PA used to advertise a lot of classic American guns in the Gun List.