For planting in northern Maryland and south central PA, which vendors should I investigate who would have a wide selection of historic iris from 1850-1920's?
Is it better to find a grower in the same zone or cooler?
i would start with "superstition gardens"
photo album - http://community.webshots.com/user/rickt103
bluej iris & argyle iris have a good selection, too, & I think there's one called shadowwood or shadowood or shadow wood.
Take a look at the list Carlos put together on his website. He tells you what kind of irises they specialize in.
Here is a link that might be useful: Brighton Park Iris links