I got my storage system way back when, that's when typewriters were the thing. It's a wooden file box that held different types of typing paper and letter head. If you ever went down to the typing pool all the ladies had them on their desks. It will hold 8 different grits of sand paper (which I buy in contractor quantities) and has a small foot print. I keep it on a shelf where it's easy to get to.
For my rolled stock I use the system that PSI has but got a better deal on the exact same one from Amazon.
I haven't made it yet but my father in law made a sandpaper sheet cutter by screwing an old hacksaw blade to a piece of wood. He put a thin washer under the ends of the blade so the paper would slip underneath it and you could "cut" your sheet paper to the size you need. Works well and you can cut any size piece of paper you need.