In general, the least expensive material will come from a local sawyer. Since you are posting from Oregon try looking for red cedar for the door cladding, a very light weight wood with excellent weather resistance. Consider making a rail and stile frame with internal bracing and simpson strap rails to stile. Space rails about 3 feet on center. Shiplap the cladding and nail cladding through the frame and clinch the nails. The door will not come apart. Be sure to place roller guides at door bottom to prevent door from being wind blown and place stop/keeper at full open position.