First, there're some shops like Anime World Star
will hold an auction once in a while, but the 
chance to win is not high whether you have tons
of money or not.  Same thing for Mandarake Zenbu's
auction.  Sometimes, some small dealers and shops
will sell some cels in some conventions, but 
mostly from average quality and so-so popular 
series.  There're a couple events in Tokyo and
Osaka that people gather to trade the top quality
and rare cels.  These people don't usually sell,
they like to trade.  But if you can offer a very
high amount and able to communicate with them
in Japanese, of course, they will give you a 
chance. 

The high quality cels are either already collected 
by die-hard fans or waiting you to bid in the 
auction.  Or if you have connection, getting the 
cels from the studio directly will be the best...