The Smithsonian Department of Paleobiology Collections database lists three specimens in their collection: PAL 174179 (hypotype - Mahantango Formation - Hamiliton Group - Frame Shale Member - Bedford County, PA), PAL 177384 (hypotype - Romney Formation - Hamiliton Member - Alleghany County, MD), and PAL 177385 (hypotype - Romney Formation - Hamiliton Member - Alleghany County, MD).
Dave at Views of the Mahantango gave me specimens of this species also. It was found in the Ludlowville Formation of the Hamilton Group of Erie County, New York (documented in this May 25, 2010 posting).
This link leads to the other Athyris spiriferoides I have found documented October 7, 2009.