Where in NYC can I buy bulk postcards for my new business?

Answer 1

The United States Postal Service has such a service you can use from start to finish - see http://www.usps.com/directmail/createdirectmailonline.htm

Answer 2

Staples
they now have a print center and will not only do post cards for you but also business cards, stamps, mailing labels, pamphlets, brochures, signs, banners
you name it they'll make it
and if you sign up for the staples reward card you'll get part of what you spend back each quarter

Answer 3

Check the yellow pages or you obviously have the Internet do a web search.