Custom paper stock size is not available online. Custom paper size is usually made by cutting down a larger stock. You can do this yourself or ask your local print shop to do this. One quote I got was $37 for 500 sheets of 12x9 paper size. In most cases, it is cheaper to stick to the two or three traditional paper sizes. For example, you can print in two or more columns (in brochure mode), and you can scale up your print somewhat, to a larger paper size, if paper size is not very critical.