New roof needed for storage shed

I have a 7'9" X 12' wooden storage shed in my back yard. Looks sort of like a miniature barn. I'm going to paint it this spring to spiffen it up, but it needs a new roof.
The old roof is tin or aluminum or something like this. It came with the house I bought, so I don't know anything about where it came from. I've googled for storage shed roofs, but haven't come up with any place where I can just buy a roof. Does anyone know of any place that just sells roofs? Or, what kind of contractor would I contact to have someone make a roof? I can't imagine that the same people who put roofs on houses would do something this small. . .and what is the best kind of roof to put on? Should I replace it with aluminum/tin or something else? It snows (in fact, it snowed last night), rains a lot and gets pretty cold here, so whatever material is used will have to deal with those elements.
The reason why it needs a new roof--it looks like someone used a giant can opener on one side of the roof--like they were trying to get in through the roof. I don't know what made the tear, but it doesn't look repairable.
Thanks for your help!

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