Any industrial metal suppliers has what you need. 1 x 2 x .125" rectangle tubing in 20' sticks and 1.5 x 1.5 angle for the seat braces in 20' sticks. Cut it, form it,and weld it on.
If you live in an area with shipyards, boatyards or heavy industrial many of these places have tons of scrap laying around, I get scrap from them to build utility trailers occasionally.