This costume uses some pre-made clothing, but parts of it were made from scratch. It starts with brown slippers, black socks (which should be dark blue, but I didn't have any at the time), white pants, and a dark blue sweatshirt. His blue tunic was made from scratch by taking an old shirt of mine, taking it apart at the seams, and tracing a modified pattern onto some poster-board to cut out the proper shapes from the cloth. Once sewn together, a bit of dark blue trim was added to the collar. The hood was made from a cap pattern I bought which was modified some to make it look like Ashitaka's hood. Two small white felt pieces were hand-sewn onto the hood. His pouch is simply a piece of circular cloth with yarn attached to the edges to make a drawstring bag.
UPDATE: I made a pair of white pants from scratch simply so I could say I made most of the costume myself. This was done by taking an old pair of pants, taking it apart at the seams, examining the pieces, and then using them to trace a pattern on to posterboard. I then used this pattern to cut out appropriately shaped pieces of white cloth and sewed them together to make the pants.