I have a Heritage Rough Rider single action revolver in 357 magnum that I usually shoot .38 special through due to the cost and sometimes availability of 357. I recently saw that some manufactures are now making cylinders that are chambered in 9mm that are interchangeable with their 357 models. I looked on Heritage's web site and it looks like they are only making 22's these days so can't find anything from them. Does anyone know if there are any aftermarket cylinders or possibly one from another manufacturer in 9mm that will safely fit my gun?

You could try Genesis Custom in Kentucky. He does custom work on revolvers but I don't know if he's done Heritage.

TK custom in Illinois also does moonclip conversions.

Seems to me for what you'll spend on custom work just go out and buy a Ruger Blackhawk with a 9mm cylinder. Might cost a bit more but you'll also have a better revolver.