have a drive round any industrial estate, if you see anywhere that has a pile of pallets or crates sitting there, its worth contacting them and asking. I once built a shed from packing crates aluminium window sections came in, each crate was 16 foot long and about 18" x 18" solid wood planks 3/4" thick. made the floor, walls roof, and even benches inside. The whole thing was waterproofed with a couple of rolls of cheap roofing felt from a reclamation yard. the biggest cost was for the nails to construct it.
Grendel