I have a die for 1 square...anything larger, I call Art Morrison. They are very reasonable and will do whatever you want...and they are very smooth on the sides in 2 or 2 by3, 2 by 4 or 2 by 6 inch.
Kind of late response, but I second the Art Morrison recommendation. I used them for the rounded corners of the flatbed on my COE truck. 2x4 tubing mandrel bent the easy way. Perfect 90 degrees on each bend, and maintained the nice rounded corners on the rectangular tubing. I actually had two big U bends made, one for each end of the flatbed. I believe they sell stock bends with around 24 inch leg lengths.