What about glassine? Available on rolls in most art supply places. I use it to separate fiber prints when flattening in a dry mount press at low (just warm to the touch) temp. I was told by a museum conservator that it's a good neutral storage material.
I don't know if they carry what you are looking for, but I've been very happy with Archival Methods in the past. No relation to them. http://www.archivalmethods.com/
you might also think about gaylord brothers, they are a supplier to libraries
or talus in new york both talus and gaylor sell book binding supplies.
if they don't carry what you need, they might be able to point you in the right direction