Each of my carts (along with it's overlay) are stored in these containers from Scraponizer. These folders are intended to hold four cartridges, four overlays, and four booklets. You'll see below that I store my booklets in a little different manner---so I'm able to fit 8 carts and 8 overlays in each. I do this but using the original plastic that comes with the cartridge. I trim that on either side and then hot glue it into the top section of the Scraponizer folder.
I then store all of these folders in this carrying case from Michael's. I believe these are regularly $8.99 each. I can fit six folders in one case. (That equates to 48carts!!!)
I store the binders in a small shelving unit I bought at Target. It was probably $12.99. Then I use a small dolly to wheel that around if I go to a crop.