To add to chipn's comments, if you can get a list 
of dealers from the convention, try to figure out 
if any of them have online shops or contact info. 
 Then I would suggest a polite e-mail to them in 
advance, telling them what kind of cels you like 
and what your price range is.

No dealer can bring their entire inventory, and 
your request might make them bring that one 
special cel you like that would have otherwise 
been left behind.  (Of course, someone else may 
buy it before you, so arrive early.)  ;-)

I've only been to one anime con, although I've 
attended it for the last three years.  It's a bit 
small, so it will probably be pretty different 
from what you will experience in NY.  All I can 
say is, see as much as you can.  ^_^

Oh, and be forewarned that anime cons can get 
pretty stinky.  For some reason, otaku are even 
less likely to bathe than Star Trek geeks, fantasy 
nerds, or even gamers.  ;-P

I hope you have a blast!

JP