Hello Bulma_gt,
There is a seller on ebay that sells storage bags his ebay name is intenseanime and he only charges shipping costs he does not add handling fees to the total.
He does have a small gallery here on rubberslug,
http://intenseanime.rubberslug.com/gallery/home.asp
Send him a feedback email and he can help you out on the bag side just give him the size item you are storing and he will pick out the right size you need.
The big thing with bags is some of them turn yellow and get real brittle over time,The acid content is not as big a thing as it used to be in the past as most plastics are stable nowdays.
Most clear plastic bags will work but if they are the kind of milky white colored ones they will turn on you in time.
Try to find a comicbook store or book store or any good size art store, Most have some kinds of bags for storage.
Itoya is the maker of most cel storage books,Here is a link to them so you can see if they have any stores near you that you can find the books in,Drop them a email.
http://www.itoya.com/Catalogs/Profolio/Profolio_html/Art_profolio.htm
I hope this helps some.
Roy