Most of the time, there is at least one booth that sells cels. 
At smaller conventions it is less common. At large ones 
such as AX, Otakon or Akon, there should be a lot of actual 
cel dealers (like Asylum, Anime Ed, Anime-Link etc). Booths 
that sell cels usually have them displayed prominently. 
Though i've run into the random dealer who had cels 
hidden away and you had to ask for them. 

You would want to check this forum (as well as the other 
"well known" cel collector forums) to find out the date of a 
meeting if there is one. 

I've been to one cel gathering in the past, and it was a lot of 
fun to meet people I'd only communicated with text. It was 
also fun to see all those lovely cels in person! 

~Tara  (who will not be at katsucon... :/