Dont buy a Bloodgood variety that is very common in the NY area. They have large leaves and internodes and only make for very large bonsai.
You can sometimes find good bonsai varieties at nurseries. otherwise, you can search for maple nuseries online.
Also Evergreen Gardenworks and International Bonsai sell bundles of seedlings.
International Bonsai takes orders starting in the winter for spring delivery
Evergreen Gardenworks usually has plants available in the late winter, spring.