From Mathew McDowell, Wildflower Island Curator at Teatown Lake Reservation
Rosedale nursery is a good place to get native shrubs and trees in particular (they have perennials as well but their selection of native herbaceous stuff isn't as good as their native woody plants). They host the Native Plant Center's fundraiser plant sale, which is a good time to pick up stuff.
I would also recommend the following local nurseries. They all focus on natives, all do it the right way, and all have incredible stock lists of stuff that can be hard to find:
https://onenaturellc.com Based in Beacon.
http://nativelandscaping.net Based in Pawling.
https://wildgardensnursery.com In Cortlandt Manor
HilltopHanover https://hilltophanoverfarm.square.site has a native plant propagation program that's only growing plants that were originally from locally sourced seeds. This is particularly useful as it helps preserve regional genetics, so these plants aren't just native they're also specifically from this area.
A bit further out but Earth Tones in Connecticut http://www.earthtonesnatives.com and Toadshade in New Jersey
https://www.toadshade.com are also awesome and worth the schlep, especially if you're looking for rarer things.
There are a lot of good, online retailers that ship as well. Some of the nurseries I've used with clients include:
These two are the really big ones. They're based in the midwest and I've been trying to pivot to buying more locally when able, but you can't go wrong with either. I've only ever had great experiences with them.
Based in Maine. They sell seed packets derived from local ecotypes primarily but their website has a ton of great info on it so it's worth looking at regardless.
New York based native nursery that ships. I ordered things from them for clients and I was pleased. They have a nice selection, especially of woodland plants and I like that they're more local.
New Jersey based native nursery that ships and works on restoration projects. Same story with Amanda's Native Plants, got things and I've been very happy so far, and they've got a really nice selection. They also specialize in local ecotypes and have a fantastic stocklist.