Why would anyone want to leave Moncton! I have to remind you, there are no Tim Hortons in NYC. heh.
Seriously though, you will need to be sponsored as far as I know. There are some career domains which are allowed to apply for a US work permit by Canadians under NAFTA, but I don’t believe that graphic design is one. I would suggest looking this up on google though. The info is buried in some legalise NAFTA stuff.
I would suggest contacting places that you would like to work at in NYC with some samples and telephone calls. If you could arrange a trip, you should probably arrange to meet some of the interested firms in person. You may want to especially hit the big firms because they will be more likely to have an HR department that handles all the paper work, and small firms may be put off to going through the process.
OR, marry a Yank.