The grate for the 18 1/2" kettle grill will fit for the top grate of the WSM. However, the bottom grate which is a "weird" size can only be ordered from Weber Customer Service (ask me how I know this )
Last week when I did the jerky, I got the extra top grate that I have on to the screws in that hold the brackets for the lower shelf. It worked great for the jerky.