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I have dealt for some years with:
Conservation Resources International, Inc.
8000-H Forbes Place
Springfield  VA  22151
1-800/634-6932
They offer a wide variety of acid free and other conservation materials.
They have an extensive catalog but do custom sizes for needed items as
well.  Their prices seem competitive.  In March I ordered 150 Mylar
polyester envelopes for maps @ 30" x 40" which priced at 2.34 ea. (3 mil),
and, 150 acid-free folders @ 36" x 48" which priced at 2.60 each.
I always ask for Livonia when I call as she has been particularly
helpful.  Hope this helps.  Chris Ziegler