One place I'd recommend is American Plas-Tech 
(http://www.americanplastech.com, used to be 
dandlproducts.com) - cheap, but they have a 
minimum order, IIRC. Other places: Noriko's Anime,
Celmart, IconUSA, Light Impressions, Talas, etc.
There are a ton of places, just remember that 
the bags/sleeves people usually refer to aren't 
actually "cel bags" in the specialty sense. ^^
The same applies to "cel books", which are just 
binder/scrapbook things. 

Cel Supplies Guide still going under revision 
and splitting with the Maintenance Guide.. Blah, 
so much work to do. ;-;