Richard - nobody seems to want to bite on your question so I'll stick my neck out there.
As for where to find the 18" SS grill grate, call Weber Customer service. I think they will be able to hook you up and for a reasonable price. If you need any additional replacement parts or accessories, order them at the same time since I believe they still charge flat rate shipping and a very reasonable price at that.
As for they why and wherefores, as far as I can tell the differences are rust and durability. SS is more rust resistant and the nickel plating, which is somewhat rust resistant, will wear off and rust will set it, making the nickel plated grates less durable than their SS cousins.
However, I am just a country lawyer stuck living in the city and there are a lot of guys on this forum with much more technical and scientific knowledge that I have and if I'm wrong or if my answer is incomplete, maybe they will weigh in and correct the misstatements or fill in the blanks.
Pat