B/c most of the dealers will have their inventory 
approximately one month before the cons; so you 
can save your time and money by emailing the ones 
who show up on the con's site as registered 
dealers. The usual ones are Nichibei, Newtype, 
and possibly Rick of AnimeLink. There might be 
more but they usually RSVP in advance. Why pay 
extra (i.e. registration) to buy the cels from 
the con when you can buy it before the con even 
starts and AVOID the senseless rush? These are 
just ways I save time and money, most dealers are 
accomidating if you email with enough time prior 
to when they begin packing away.   Good luck.