I found this great list of upcoming native plant sales on the Chesapeake Conservation Landscaping Council Website.
Many of them are already past, but there are still some coming up and I'm sure they will be adding more as they hear about them.
June 3 & 4 – Millersville Native Plant Sale
June 25 – Nanticoke River Jamboree
Handsell – Vienna, MD
August 27 – Irvine Native Plant Seminar & Sale
Irvine Nature Center – Owings Mills, MD
September 9 & 10 – Environmental Concern Fall Native Plant Sale
Environmental Concern – St. Michaels, MD
September 17 & 18 – Adkins Arboretum Native Plant Sale
Here's the link to the list, so keep checking back: Chesapeake Bay Region Native Plant Sales