I know Noriko 
(www.darkharbor.com/trades/html/cels.html) sells 
the 'classic' envelope-style w/ sticky flap 
polypropylene bags in 11" x 14".  You could also 
try celmart.com if you don't mind paying for 
international shipping, and they do have several 
different sizes for polypropylene bags.

iconusa.com sells polypropylene sleeves (no 
flaps, both ends open) in 11" x 14".

As for the 11" x 14" sleeves/bags, they should 
fit fine in most 11" x 14" Itoya's and other 
portfolios, but there are some Itoya's out there 
that are manufactured a few mm off, so 11" x 14" 
bags may cause bulges.  (One of the recent 
Itoya's I bought does this where the rest don't.)