Try Dave Lambeth
I don't know of anyone offering a complete system in stainless, but Motad do stainless down pipes that connect to the original silencer and some aftermarket ones, including the 'stainless' Laser Produro system.
This is the system I have fitted to my 1988 3AJ (twin headlight fairing) Tenere and my girlfriends 1991 XT600E. The Laser Produro system is described as stainless, but the baffles are made of mild steel and will rust away over time.
I think the concensus is that there is a better , totally stainless steel silencer available, possibly a Preditor but I'm not sure.
Dave Lambeth is a good person to obtain these from, do a google search and you should find him and he will be able to advise. When I got our systems in 1999 the got the parts quicly and a little cheaper than M & P, and would have supplied the better silencer if I hadn't been in such a hurry.