The best one I know of is Sprainbrook Nursery:
Sprainbrook Nursery Home
(They have a Scarsdale address but I believe they are really in Edgemont.) It's a big place with a very good selection and very knowledgeable staff. Also, they are very much in favor of organic methods which I think is always a plus!
I am in full agreement with you about perennials! Some that grow well in southern Westchester and require little upkeep are day lilies, black-eyed Susans and any type of daisy. My Dad lives in Scarsdale and his whole garden now is pretty much made up of day lilies. They come up faithfully year after year. Peonies also seem to do well although they require a bit more work--you have to tie them back and prune them a little.
We have also had surprisingly good luck with roses, which some say are high-maintenance, but ours seem to grow just fine with adequate sun and water.
Good luck!