I've started storing my unmounted rubber stamps in a notebook - in standard plastic protective sheets. (To see how I turned the unmounted rubber stamps into cling rubber stamps, check out this previous post.)
The first unmounted stamps I got were those I purchased from Stampscapes. At the time, I had purchased a mix of cling rubber and unmounted stamps. Some of the cling rubber ones were in sets that came adhered to hard white plastic sheets. I decided not to keep them in the sets, but rearranged them so all trees were together, all animals & birds were together, etc. In doing so, I was able to make room for the unmounted stamps too.
However, I discovered that storing them this way made it difficult to see what some of the stamps were! So, I stamped a cover sheet that had the stamp in approximately the same place as on the white plastic. This actually served 2 purposes. Not only could I easily see what I had and find it, but I could also tell what stamp was missing if they weren't all back in place! I then stored the white sheet plus the cover sheet in a standard 8 1/2" X 11" protective sleeve and put them all in a zippered binder.
However, I discovered that storing them this way made it difficult to see what some of the stamps were! So, I stamped a cover sheet that had the stamp in approximately the same place as on the white plastic. This actually served 2 purposes. Not only could I easily see what I had and find it, but I could also tell what stamp was missing if they weren't all back in place! I then stored the white sheet plus the cover sheet in a standard 8 1/2" X 11" protective sleeve and put them all in a zippered binder.
Some time later I was given a bunch of other unmounted rubber stamps that I had to figure out how to store. I didn't have any more of the white plastic sheets. I knew I could buy some, but prefer to spend my money on stamps rather than storage! :) Then I thought of some of the plastic packaging I had saved when I bought some of the larger Spellbinders dies. I cut it down so it would fit in an 8 1/2" X 11" protective sleeve and stored my newly prepared stamps on this plastic sheet (inside the sleeve, inside a notebook).
I actually like this BETTER than using the white sheets because I don't have to make the cover sheet to see what I have - I can see it right through the clear plastic sheet where I stamped the fun foam as I prepared the stamp for use with a clear acrylic block! (I expect one could use an acetate sheet for this - but it might not be as sturdy.)