I got my signs at the surplus pile at the state highway engineer's yard. Aluminum signs are very hard to recycle because the coating on the signs is expensive and difficult to get off, therefore making them unprofitable to melt down. Old signs are sometimes just thrown in a big pile and forgotten about . . . all you need to do is ask. I found three the same size, 15" x 21".
Don't steal signs. They are expensive, paid for with your tax money, and you could potentially create a dangerous situation by removing an important sign from the street.
All the wood was found 2" x 4" scrap that I milled down to about 1-1/4" square. The only expense was the screws, 3" drywall or similar, that ran me about two bucks for half a pound.
You will need these materials:
Approx. 16' of wood per stool
3" drywall or wood screws
You will need these tools:
Miter, circular, or jigsaw
Impact driver and/or drill