Sorry, I'm not sure what it is you haven't found? Sub rails or source causes for the rust. There can be many areas from where water gets into the rails. Check under vents under the windscreend wipers with a light and a mirror. Also there are drain holes in the rails if they are blocked then water can get in through where the rail meets the firewall but will not drain out. If water can sit in any spot then rust is inevitable.

As for finding new rails I don't know. I had to fabricate a whole new one with the help of the metal bashers where I work. I guess any engineering or sheetmetal workshop could make you one.