Where to purchase alum. siding for homemade RV?

When we build site office trailers we use shallow ridge steel roofing, available at almost any lumber yard. At the top we use the same metal trim used for capping the parapet on a flat roof building, but in 4" size.

Be careful using residential vinyl siding, it needs to nailed loosely in order to be able to expand and contract with changing temperatures. This means it's easy for it to blow off in transit.