Maryland, Virginia and PA for sure. Fair Hill International
Most PA locations are in Bucks County around the available facilities like Bucks County Horse Park, Radnor Hunt club and more. Sorry my brain just stopped working, too much algebra. I don't get to VA often, but Upperville is a good place to start looking.
There are tons of big name eventers with their summer locations in PA and MD.
Phillip Dutton, West Grove, PA.
O'Connors are in Virginia.
Kim Severson Virginia.
James Wofford Virginia