The cel vendors at conventions are nine times out 
of ten going to be the cel vendors you see 
online. ^_~ Try a larger anime convention in your 
area--- they have a larger likelihood of having 
cel vendors present. 

Trying for a con next month isn't practical, 
though. The major con season tends to be late 
spring through early fall... Oni-con is next 
weekend, but I doubt there will be any cel 
vendors there... Itzakon is in November, and most 
definitely will not have cels... nothing happens 
during December... Ushicon's in January and I 
believe there was only one cel vendor last year. 
(Those are all Texas cons, and I believe all are 
under the 3000-attendee mark.)

http://www.fansview.com/consked.htm has a list of 
the 2005 con season, where you might be able to 
plan ahead further in advance for a large con 
near you.

If you do a seach in the archives, there have 
been some past posts about cel vendors and 
conventions that have been pretty thorough. But 
it all boils down to--- don't go to a convention 
just for cels, or you'll probably be disappointed 
in price, variety, and quality. Go to a 
convention because of the anime, because of the 
guests, because of the panels, or because of any 
of the myriad things that cons have to offer... 
but the cel selection is probably not one of 
them. ^_^ You're more likely to find better cels 
in the private binders of attendees than in the 
dealer's room for sale. ^_~