Any straight, dry, standard-sized 4x4 or 6x6 posts from a local lumber retailer can be used. Cedar and Redwood are two of the most popular choices for pergola projects, but pressure-treated wood (least expensive) or tropical hardwood such as Ipe (most expensive) can also be used. Note: lumber dimensions for standard 4x4 wood posts are 3.5" x 3.5" and standard 6x6 wood posts are 5.5" x 5.5".