Cels Forum

Your local comic book store is usually a good place to start.
However, if you can't find them there, go to
American Plas-Tech at http://www.dandlproducts.com/
(This was posted on this bulletin board before.)
I have purchased from them, and they are quite reliable.
But, remember, you pay shipping costs, which is by weight
and can easily exceed the price of the bags themselves if you are 
not located near to California.
You want to purchase polypropylene bags.  Sizes that will work are:
Sheet music (9.5" x 12.25")
Treasury Comic size (10.5" x 13.5")  (CB116 for a pack of 100 at $3.60)
Life Magazine (11.25" x 14.25") (CB118 for a pack of 100 at $5.25)
Sheet music & Treasury size work fine for almost any cel.
You only need the life size for oversize cels and large backgrounds.
You can buy bags that are resealable and ones that are not.
I'd recommend not.  Use tape instead if you feel the need to
seal the bags.  The resealable always seem to sealing themselves
at the most inopportune moments.