I was frustrated in my attempts to find suitable material for gutters (evestroughs?) Val suggested half round and I had thought of that too, but at least the gutters I'm familiar with aren't that shape. I checked out U channel and eventually selected one, tho that's not correct either. I can't recall the size I used but it would be too large for a home or small structure, mine went on a large building and I thought it looked ok. I did buy a set, it may have been Rix, I don't recall, but for a structure 29" long, it was just silly! It would probably be your best bet for smaller buildings. Downspouts are easy if you want round ones, just use rod or tubing. But for the rectangular corrugated aluminum ones, I don't know of a source. If you have the patience, you could use strip styrene and use a straightedge to scribe the corrugations in. That would be fun.
Good luck!