Discount wall paneling is sometimes factory seconds or panels that have some other defect. Dealers often discount paneling very deeply to make up for the defect. It is important to make sure to check the panels for any major structural issues such as splits, warping, or dents.
Habitat For Humanity ReStores and other surplus building supply warehouses are good places to look for discounted panels. Sometimes contractors will sell leftovers from projects via ads on Craigslist or eBay.com.
Home improvement stores such as Lowes and Home Depot offer a contractor discount with a minimum sized wall panel order. Sometimes stores such as Home Depot will beat the price of any competitor if shown a lower price than what they offer.