It's very hard to predict whether there will be 
any cels for sale at a convention.  The 
convention sells tables to the dealers, and the 
dealers buy tables.  If cel dealers buy tables, 
then there will be cels.  If all the tables are 
bought by bootleg CD dealers before any cel 
dealers get into the act, then it will be all 
bootleg CD's, and no cels.

The best thing is to get a list of the dealers 
who will be represented at the con, and determine 
whether any of them are cel dealers.  Then decide 
wisely based on the information that you have.

Now, those always-true generalities aside, and 
moving on the the not-always-true specifics....

There is a certain major cel dealer who buys 
tables at most major cons.  I won't name him, 
because it's not my place to either be his 
marketing arm or to interfere with his marketing 
plans.  But I will say that he was at Otakon last 
summer and at Anime Boston this spring.  He's 
likely to be at anu major con.

I may never buy another AMG movie cel again, 
because he showed up at Anime Boston with a huge 
stack of AMG movies cels, priced way below what I 
thought that the eBay or YJP market would 
charge.  I and mine walked away from Anime Boston 
with a large stack of AMG cels at what we thought 
were great prices.  We paid 25% to 40% of what we 
thought each cel was really worth on the open 
market.

I also bought a Nuku Nuku opening cel at what I 
thought was a great price.

Also, we have NEVER seen a decent Magic Users 
Club cel for sale, at any price.  We've bought a 
couple of douga sheets of MUC, but that's about 
it.  The few cels we've seen have been of 
people's feet and butts and so forth.

At Anime Boston, we found a really excellent MUC 
cel for sale in the 2-digits, and of course 
immediately bought it.

On the other hand, a lot of his cels were 
seriously over-priced.  We passed on those, but 
if we had taken those as the norm and given up 
looking, we would have missed the exciting 
bargains.
 
So, cons are kind of a gamble.  You can go, and 
no cel dealers show up.  Or, like our Anime 
Boston experience, you can go and come home with 
a huge pile of very-underpriced treasures.

Your mileage may vary....