The University of Connecticut runs a simple database that allows you to find plants that grow well in Pennsylvania and even lets you pick flower colors.
Pennsylvania Native Plant Society provides lists to where to find native plants in Pennsylvania. Many of the nurseries listed will ship to your home.
To buy plants in person, native plant nurseries in or near the CRC watersheds include:
Redbud Native Plant Nursery
Yellow Springs Farm
There are also native plant sales to benefit local non-profit organizations.
The Brandywine Conservancy holds a native plant sale every May in Chadds Ford.