18.5" stainless steel cooking grates are getting hard to find. Here's one source for the top grate:Where can I find the stainless top and bottom grates? Also, is the coal grate the same size as the newer models?
https://www.amazon.com/exec/obidos/ASIN/B00TEBDRWY/tvwb-20
I'm not aware of a stainless steel version of the bottom grate at this time. Even the standard version is expensive because of its unusual size. Here's a link for one source:
https://www.amazon.com/exec/obidos/ASIN/B013KIGPZC/tvwb-20
And yes, the charcoal grate remains the same in new or old WSM versions.
If you decide to go with standard cooking grates, it wouldn't be totally crazy to order all three pieces from Weber direct at 800-446-1071, they'll ship all by FedEx Ground for a flat fee of $7.