Used can be hard to find. Some dealers have coversheets or cancellations they'll sell at a discount. Coversheets are extra pieces, often shorts a/o different colors, that are used to protect ordered roofing during transit. Craigslist or local want advertizer sometimes has some. MikeD74T
Look in your local farmer's bulletin. I often see used "chicken house" tin in the Alabama one. Or craigslist. Usually about $1.00 a foot. You can buy new 26 ga. galvalume for $1.80 a foot.