I don't know who eworld is but this who manufactures those.

http://pro-werks.com/categories/843/

If you look close, you can see a seam on the inside of the donut. Those are made from two stamped halves and then the outside is welded and finished. You finish the inside weld when you cut it apart.

I've been using Columbia River for most of my mandrel bends. They usually offer two bend radii per tube size. Large radius stuff is harder to find. You might try Summit, they have some 6" CLR bends depending on size. I just built a set of pipes for my sprint car where I got mild steel 1-7/8" by 3" and 4" CLR from Columbia River, and 6" CLR from Summit. If I remember correctly, the 6" pieces were made by Doug's Headers.