The bonanza.com link is exactly what I had in mind, however looks like they just have one in stock.
I did locate something that looked promising on
Amazon.com, those are listed as 'poly' protectors, claim to be archival (dont they all?), but am not sure I trust that. The Averys I am using say they are but they have the oily feel of something that could hurt stamps and covers.
Strange these used to be very easy to find, guess no one uses them anymore. I do, for my US collection on mystic Heirloom pages.