Homasote. This is technically a compressed fiberboard product that is sometimes used for soundproofing and insulation. Great for large bulletin boards because it takes pins really easily, you can cover it with fabric, paint it, or frame it. It's about $12 for a 4 foot x 8 foot sheet. - Cheaper than cork sheets at Hobby Lobby.
How to Make a Homasote Bulletin Board - Decor Adventures
DIY Home Office Bulletin Board from a huge piece of foam insulation board. Attach the fabric to the back, using large pieces of clear packing tape (or duct tape or flat thumbtacks. whatever floats your boat).