1/2" steel is pretty serious stuff. Just to give you an idea, a 12"x12" sheet of 1/2" steel usually weighs about 20 lbs. Because geometry plays a huge roll in overall strength of a structure, we would need to see how you plan on using the sheet metal, otherwise it would be hard to make a recommendation on what thickness you need. As a reference, the chairs that Nxakt posted are likely 1/8" aluminum (anyone else know for sure?) and they look to be strong enough. Are you planning on cutting a strip of steel and bending it into the “U” shape, or cutting the “U” shape from the flat sheet of steel?
Using recycled metal is definitely not a wild goose chase. Here are some things to consider: If the chair is to be a one-off product, then you can find your material anywhere. If the chair is meant to be designed for mass production, then you need a constant source of material. The best option then would be to find outdated metal objects and figure out how to use them for your needs. If you couldn’t make that work, then you would probably need to use new materials.
If the chair is to be a one-off product, you could easily source your material from a metal scrap yard. It would be recycled. These places know the value of scrap metal, and will charge you accordingly based on weight. Junk yards and resale shops are another good place to look. Often they base their pricing on the perceived value of the “object” instead of the market price of the material. This means you can sometimes get some good material for less than you would pay at a scrap metal yard.