This is what I put together for my 12x12" sheets of cardstock. I got the shelves at Target, but I cannot remember if I got one or two boxes of shelves. Anyway I know I paid $20.00 for the shelves. I put the shelves together with zip ties. This way you can space the shelves according to how wide you want them. I first made a square minus the front, and then added my inner shelves. You can use as many zip ties, as you need in order to make them sturdy. After you get your first cube completed, them you can build on that. For the second unit you will not need a bottom, because you will already have it from the first cube. If you need further explanation just pm me. Hope this makes sense.
Lorraine
The first picture is of the completed shelf unit. The second is what the individual shelves look like. Sorry for the dark pictures. It is raining like nuts here.