Charcoal and Pastel Drawing Storage
I would lean toward using acid-free tissue paper.  It comes in all sizes and you can cut out sheets large enough to separate your drawings.  I think this is similar to what my sisters use. 

Of course one of the things that could help preserve them is spray a fixative on them, to help keep them from smudging.

Last if you need a physical container, there are plenty of different shaped plastic boxes, many big box stores have different options for $5-10 a piece.  Combined with the acid-free tissue paper you can store quite a few drawings in a safe location away from the elements, dust etc.