As far as I know, no one makes that part. It is specific to the S-system. As for a the real steel part (ARMS S.I.R. # 46 SC) it operates quite differently than the Marui in regards to the hand guard and is not interchangeable.
I too encountered the same problem after I decided to upgrade and decided to start from scratch with a metal M4 body and real steel rails, pics:
http://www.airsoftcanada.com/showthread.php?t=31534
To help the S-system, I took the rail apart and used a flat, textured metal colored spray paint (originally for use with plastic lawn furniture) and a very light pass over again with a shiny metallic spray to give an overall flat with metal fleck look. Came out good, comparable to the ARMS on my other gun looks wise. I also filled in the trades with a oil pastel, then sprayed the body with a flat varnish to kill the plastic sheen. Worked real nice, but has a tendancy to scrape off. I just give a little shot where necessary to keep it flat from time to time.
