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
Millersville, PA
http://www.millersvillenativeplants.org/vendors
June 25 – Nanticoke River Jamboree
Handsell – Vienna, MD
http://nanticokeriverjamboree.com/
August 27 – Irvine Native Plant Seminar & Sale
Irvine Nature Center – Owings Mills, MD
http://www.irvinenaturecenter.net/images/native_plants.pdf
September 9 & 10 – Environmental Concern Fall Native Plant Sale
Environmental Concern – St. Michaels, MD
http://www.wetland.org/nursery_retail.htm
September 17 & 18 – Adkins Arboretum Native Plant Sale
Ridgely, MD
http://www.adkinsarboretum.org/native_plant_resources/plant_sale.html
Here's the link to the list, so keep checking back: Chesapeake Bay Region Native Plant Sales