Several depending on the bend radius you need and what's required/acceptable in terms of finish...
Short vid demonstrating a simple former that purposely deforms the inside face of the bend which helps stop the sides wrinkling/kinking, the outside face will deform ending up concave... http://www.metalgeek.com/archives/2011/04/08/000121.php. More info here, http://www.metalgeek.com/archives/2005/05/01/000047.php Can also bend hot with simple forms like the mdf/wooden jobs (need to stick a strip of metal to working face of a wooden form to protect it from the red hot steel) used for large radii in the second link or a chunk of heavy wall pipe. Again bend radius alters things... typically get less cupping of the inside and outside faces but the sides will bow out some instead
Should be able to get that rolled in a proper ring roller, the rollers themselves would restrict the side faces from bending out. Inner of it will deform more than the rest.
You could probably make a bender to do it, long arm inner former and guideto pull it rount the former, a pit like a conduit bender. Used to do this a lot with 16mm box.
If you have the ability to make something yourself, you might consider making rotary draw style of bender. These vary from the type mentioned as they draw the tube round the former rather than having the tube compressed around the former. There is little difference in producing either type, in fact a rotary draw style is easier as you do not need a guide, as in a typical conduit type bender.
They also give better results. Lots of vids on you tube to get an idea of the difference