You're in NY? Have you tried Stock Building Supply? Go in your phone book and look up wholesale building supplies. That is where we got some of ours. Rest free on the side of the road or on CL.
Its not so much "roofing' as it is metal sheets. Any home building place should carry it. They are commonly used to make "machine sheds" here in Minnesota but lots of people are now switching to putting it in the roof of the homes and barns instead of shingles.
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.
Here in Mpls, it's available at Home Depot, Menards and Lowes, but only by special order. Priced around $40-50 per sheet. I don't know the name of the "pole barn" stuff, but that's what I have on the roof of my run -- and I LOVE it. Mine was salvage, but I know that it can be ordered in all sorts of colors. All the major big box stores can get it by special order.