I'd recommend trying www.anipike.com  
Scroll part way down, and they've got a list of 
links to convention pages, and a listing of the 
dates on which they occur.  There might be a 
better one out there (I can only get to one con, 
myself, so I usually try not to depress myself 
by looking at all the others ;) but it's a pretty 
useful resource, and a good starting point.