buying native plants
sourcing plants
Finding native plants for sale is hard. The demand is just greater than the supply and the "problem" is getting worse. Here is a map of nurseries I am familiar with that have a particular, or exclusive, interest in native plants. Note, not all plants sold as "native' are native to this area and some plants sold as natives are actually cultivars. The Native Plant Trust is the go to reference for what is native to the various plant areas or "ecoregions" in New England. In Rhode Island we are in a plant community area known as the Northeastern Coastal Zone.. Here is a list from their Garden Plant Finder of plant species that are truly native to Rhode Island.
When I choose plants for a project I make sure there is a source for them.
truly local plants
The best ways to get plants is to grow plants from seeds locally (provided that it is done in a responsible manner.). The federal guidelines for how to do this are described in the Seeds of Success Manual. Planter's Choice in Connecticut sells plants grown from locally collected seed through the Ecotype 59 project. The Rhode Island Wild Plant Society also is also doing similar work through the Reseeding Rhode Island which is a few years behind that effort but intent on providing more local plants from sustainably collected seeds at their plant sales and beyond.