There are several on Yahoo Japan at the moment, 
and the last time I checked there were a couple 
on eBay.

Warning: If you're looking for Kenshin himself, 
be prepared to pay big bucks.  The least 
expensive of the cels I have of him was $200, 
and a couple of fabulous hanken cels sold on 
Mandarake recently for around $1000.  

If you're looking for other characters, they are 
much more available and often less expensive.

Good luck!