It is easily found OTC in some areas, special-order-only in other areas. You want to ask for "corrugated metal roofing" or "corrugated metal siding, you know, like you use for a pole barn". If your hardware stores can't order it for you (talk to the customer service or contractor desk, NOT the 19 yr olds working the floor!) then your feedstore or farm store quite likely can.
Price IME is about halfway between the PVC plastic roofing and the polycarbonate plastic roofing prices -- last time I checked, here in Canada, something on the order of $40 for a 2x12' sheet.
Good luck, have fun,
Pat