Homosote is recycled paper pressed into a 1/2"X4'X8' sheet. It is used as an insulating sheet in some types of home construction. It also can be covered with fabric and used as a bullitin board. Most lumber yards, Lowes, or Home Depot should carry it.
MDF-(medium density fiberboard) is not the same thing. It is used in the manufacture of most RTA (ready to assemble) furniture. It is considerably more dense than homosote; and I wouldn't suggest using it for a building board unless you plan on pushing your pins in with a hammer. It would make a nice workbench top, however. As long as you don't get it wet; it will swell up a lot!