According to the U.S. Environmental Protection Agency, native plants don't need as much care as exotic non-natives. They've spent thousands of years learning how to adapt to the northeastern climate, so they don't need as much fertilizer, water or pesticides required by non-native species. In addition, they're an important source of food for native animals and insects, in addition to providing shelter for them. Planting native plants also attracts more birds and butterflies, and encourages a better mix of beneficial insects.
The U.S. Fish & Wildlife Service urges people not to collect native plants from the wild, unless an area is going to be developed. Even then, you'll need permission from the landowner. Plants collected from the wild don't tend to transplant well. This applies to seeds, as well. In most areas, it's not legal to collect seeds from public or private lands unless you've received permission from the landowner. When collecting seed, never take more than one-third of the available amount.
Some plants are sold as "natives" because they're native to the U.S., even though they're not native to the northeast. When shopping for native plants, always ask if the species and the variety you're interested in buying is native to your area. You should also inquire about the original source of the plant you're interested in buying. If it's not from your area, pass it by, even if the species itself is native to the northeast. That's because local varieties have genetic variations. A plant grown in Maine may flower at a different time than one grown in Rhode Island. Preserve genetic variations by only planting local varieties.
Several websites maintain databases where you can search for places that sell native plants of the northeast. The Ladybird Johnson Wildflower Center lists native plants for various types of purposes, such as those that are deer resistant or those that attract moths and butterflies. An online database allows you to search by state.
PlantNative is an organization that works to promote the use of native plants in mainstream landscaping. Their website includes a list of nurseries that provide native plants for the northeast, and for other states as well. They also provide information about nonprofit and municipal groups that can help you learn more about using native plants in landscaping.
The New England Wild Flower Society is considered one of the leading plant conservation groups in the country. They operate Garden in the Woods, which is located in Framingham, Massachusetts, as a public botanic garden and center for study. They also operate a native plant nursery in Whately, Massachusetts.
The USDA Forest Service provides a 32-page list of nurseries that sell carnivorous plants, ferns, grasses, herbaceous and woody plants. Although their list covers the entire eastern U.S., not just the northeast, it's easy to find information for the northeastern region by looking at the addresses.