Just see what your local hardware store has (talk to them) -- it's irrelevant what things cost (and what's available) in other parts of the country
Most over-the-counter metal roofing comes in 8, 10 and 12' lengths and depending on the manufacturer a sheet will have an effective width of 2' or 2.5' (that's
after allowing for the overlap between sheets).
But remember your roof needs overhang on all sides, especially on the lowest side(s). Don't build a roof that's nearly flush with the walls - I'd say 6" is sort of a minimum overhang and more is better. Remembering of course that you need WOOD constructing the overhang - you don't want more than an inch or few of unsupported roof tin at any edge.
If the building itself is 4x8, and this is a shed roof sloped across the 4' dimension which is what it sounds like, then if the roofing you have available is 2' working width, the best thing would probably be to have a foot of overhang on each (short side) and not quite that on the high and low sides, meaning the roof surface itself would be 10 x 6. This would mean buying 3 12' long panels, cutting them in half, and having a half-piece left over. (Sell, give away, store for future use, or use to make a crude 'porch roof' over the pophole to minimize mud and blown-in rain). Not super easy or super efficient but it will give you a good result. If they're 2.5' width pieces, then you would buy 2 12' long panels, cut them in half, and not have leftovers.
If you don't want to be cutting metal roofing, then you will either have to design your roof so it is 8' long, or find used roofing that someone else has already cut to a useful length for you (not really recommended -- recycled roofing will seldom be really water- or weather-tight), or use plastic rather than metal (since the plastic stuff, like Palruf or Suntuf, can be cut with just stout shears).
Whatever you use, metal or plastic, make sure to use gasketed roofing screws. Gasketed is essential for waterproofness (they come with sort of a metal washer type thing, under which is a rubber washer type thing - ask the hardware store guys) and screws hold way better than nails as years (and strong winds) go by.
Good luck, have fun,
Pat