There are so many great florists in NYC you can hire to do your flowers. If you are watching your budget, artistic, or love the idea of creating your wedding flowers you can do it yourself. There are great videos on youtube that can show you how to make bouquets and all other flowers for your wedding.
Link for a video on youtube - http://www.youtube.com/watch?v=ieP97FqZo5U
NYC has a large area called the flower district that you can go to get wholesale flowers. It is in the vicinity between Sixth and Seventh Avenues from 23rd to 26th Streets.