I keep my unmounted stamps in a binder.
I think I just looked up something like "unmounted rubber stamp storage" online and found the solid white hard-plastic pages. The first set of stamps there is on a clear plastic page- they actually came on it as did the smaller set you can see through it. The only problem is that sometimes the acrylic stamps won't stick to the pages well and they actually fall off of them when you turn the pages. In the back, I used a 3-hole binder page that probably is meant to hold 9 baseball cards in a 3x3 arrangement. I keep my little $1 spot types of acrylic stamps in those- they work REALLY well for that. Just keep the backing paper on them so you can easily double side it.
Overall, I do like the binder since they're all right there. If I had more, it may not work so well since they're not really organized. by any theme, just pretty much in the order I get them or where they fit.