The 18.5 WSM charcoal grate is an odd size so don't plan on finding a replacement at your local Lowes or HD. Unless it is totally falling apart, it should be fine. One thing that a lot of us WSM users do is that we buy a charcoal grate made for the 18.5 inch kettle and place it on top of the WSM charcoal grate oriented in the opposite direction. This is especially helpful if you plan on using lump as it will prevent a lot of the small pieces from falling through the grate. It basically does the same thing as putting expanded metal over it or using a charcoal basket but it is just a lot easier to come by.
WSM, 99 & 09 Performers, 2-OTG, SJG mini WSM, Summit 420, Q100, Akorn, and Schwenkgrill