There are many stores and websites that offer wood panels in various sizes, including 4 x 8, such as Home Depot and Lowe's. Rustic paneling is available at Mantelcraft.com, and eBay.com offers multiple paneling choices.
EBay.com offers 4x8 wood panels from different types of lumber, such as bamboo or cherry. A cheaper option is plywood, which is comprised of several individual layers of veneer glued together. Veneer is a thin sheet of wood that was mechanically peeled off a log, just a paper towel peels off the roll. Plywood is replacing solid wood in many instances because it resists splitting, cracking and warping, besides being more cost effective.