Usagi's House had them a while back, but I am not 
sure if they still do.  They were made of mylar, I 
think and are nice and thin, like the kind you get 
with cels from Anime-cel.com.  Otherwise, Noriko's 
Anime has the thicker ones that are made of poly-
whatever (sorry, I don't know the name of the 
plastic). 
Both kinds have the sticky strip at the top so you 
can seal the bag shut and open it later if you 
want, though, over time the mylar bags sticky stuff 
solidifies a bit and then you can't open the bag 
anymore, because it will rip.