Archivists swear by Hollinger Metal Edge, the leader in this field since 1945. If they say it's acid-free, you can count on it. To be completely honest, I get nervous when genealogists tell me they buy acid-free supplies at Walmart or other retail outlets that aren't in the preservation business.
Products that fit most family history needs include:
If you've heard my presentation on organization, you know that this is where you should be spending your storage dollar, rather than on elaborate binders, color-coded systems, and sheet protectors from discounters and office supply stores. Please store your original and vintage materials appropriately in trusted archival supplies like the ones from Hollinger. Light Impressions also supplies quality archival products, but they seem to be ill-equipped to handle small personal orders. If you need your archival supplies and don't have months to wait, then try Hollinger Metal Edge first.
And, as always, I have no financial interest in either firm.