I was out at our local big box home improvements store today and found that ridge roofing is now available as a special order product. It is offered in 10 different colors and has all the trim materials. The store can order the panels precut to the inch at a very attractive price. One tip is to first choose the roofing and then make the roof length fit the width of the panels. The panels I saw today are 38 inches wide. Three panels would give you 104 inches. If you don't want to have to cut the panels to make the 8 foot as called for in the plans, consider building the coop to fit the 104 inches. Alternatively, the metal panels can be cut easily with tin snips or aviator snips.
The matching screws sold with this roofing product are also expensive. You can substitute a generic screw with a rubber washer available at the home improvement stores. Here in Madison we have Home Depot and Menards. I have not checked to see what Lowe's has to offer.
Again if you find the metal roofing too pricey, consider using an asphalt corrugated roofing material. It won't look quite as finished, but the chickens will surely not complain.