I know of a few sources;
Archival Print and some negative storage bags;
Bags Unlimited,
www.bagsunlimited.com
Archival storage boxes, also some print bags (Not to mention Library supplies- their main business)
Brodart Inc,
www.shopbrodart.com
Frame Destinations, they advertise here
General archival supplies, boxes, mat board, etc.;
University Products,
http://www.universityproducts.com
My wife has been buying stuff from Bags unlimited and Brodart for quite a long time for her bookstore, I recently bought a clamshell storage box from them which looks to be just fine, and was resonable cost.
I've used Frame Destinations along with many others here in APUG land.
I've not purchased from University Products, but they are a well regarded supplier here in the North East (U.S.), located in Holyoke, MA and do mail order via the web, etc.