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
Step 1: Framin'
To make the top of the seat square, take the short dimension of the sign, in this case 15", and make that the length of the top piece of your "A" frame.
Cut the legs to your desired length with a 5 degree miter to get a nice taper effect. A rule of thumb is that a stool should be a around 8 inches shorter than the surface where you will be sitting. So, for a 42" bar, make your stool 36" tall; for mine, designed to match up with a standard 36" kitchen counter, I made them 28".
Make the miters at each end of the legs parallel to one another so the stool will sit flat. Measure up from the floor 6-10 inches for the crossbar brace.
Attach everything with one long screw and a healthy dose of glue. Pre-drill for your screws with an 1/8" bit to prevent splitting, and counter-sink your screws so the heads are flush.
Knock down all the sharp edges with some sandpaper.