You can purchase damaged sheetrock at just about any place that sells sheetrock. It's the nature of the beast for some of it to get damaged in handling. As a cabinet builder I would highly recommend AGAINST using it for backing of a bookshelf. Your bookshelf will never be square and can't be built with any precision at all. They call it wall board for a reason, it's for walls. If you need an inexpensive product for a back use tempered hardboard or masonite as most folks call it. I've been in and around the construction business most of my life and I've never seen drywall used as a backing for a bookshelf.