These are the ones I use for this very purpose:

which I get from
Amazon. They are poly pages, "acid free" and "archival".
The brand name is C-LINE and you can usually get them in boxes of 100 for about 16.00-17.00 USD. They come with a black paper insert that I usually use for another purpose or toss...
What you are looking for are "traditional" page protectors.
John