I don't know about 'Cheap', but I'd look into old conveyor belting from a factory or mine or quarry.
Lots of times they will give the old stuff away just to get someone to haul it off.
Someplace that repairs conveyors will have belting laying around you can scarf up for little or nothing.
You might look for an old barn or house being taken down that has metal roofing.
Since you aren't trying to use it to keep water out of the house, the nail holes won't matter and you would be able to get that for cheap.
Metal sheeting at the recycled home places is dirt cheap.
They take it off buildings, roofs, ect. and try to resell it instead of scrapping it, but most people want it for roofing, so they don't want nail holes.
Usually sold for scrap weight when you find it, so it's very cheap.
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If you need insulation from the cold, the Styrofoam board from the lumber yard or even straw bales (which are more expensive than Styrofoam now and rot)
Will work for insulation behind the conveyor belting or tin outside.