Thanks for the comments. I finally figured out what I need. The grate size that fits into the opening is 18 x 11-3/4. A grate from a Genesis 1000 looks like it will fit. Now comes the hard part, finding a single grate!
MikeZ
There is a guy on eBay that sells stainless stell replacement grills. You may be able ot buy just one. Make sure your measurements are correct.
You could always buy the cast iron set and sell only one. There has to be a memeber here that has one of those grills that needs replacing..
Here is a website tha sells the Cast Iron replacement grills. Weber Grill Parts