It is important to point out that there is no standardization in the outdoor cooking industry. Just because two grills appear to have the same measurements doesn't mean that the cooking grates can be swapped between them. Further, different grills have different ways of holding their grates in place. Always check with the manufacturer of the cooking grates to find out if their grates will fit your grill. Most aftermarket makers will know this information. Manufacturers of grills probably won't, but they will have exact measurements for their cooking grates.
Shop for Anodized Aluminum Grill Grates
Stainless Steel Plate
Cast Stainless Steel
Stainless Steel Rod
Porcelain Coated Cast Iron
Bare Cast Iron
Take a look at ManGrate for more information.