You won't find it at your local home depot. You have to go to a Drywall Supply store. One that caters to Drywall Contractors and Steel Framers.
Just so you know, they manufacture drywall in various widths, most common are 48" (obviously) and the next most common width would be 54". As for lengths, you can custom order it as long as you want. Sometimes we order 16' sheets to avoid butt joints. Costs more money, it's a ***** to deliver and install but worth it on very large projects.