These grill grates, and others to fit most round charcoal grills, are sold here.
Two optional accessories are the hot stone insert, and the chicken roaster insert.
These grates are said to do a much better job grilling your food than the regular steel grates. And I like the griddle feature!
Lodge also has a cast iron grill grate; It doesn't replace your steel grate, but rather sits on top of it. It's spaced narrowly enough so you can grill fish and veggies without it falling through.
Since cast iron rusts, do not leave these grill grates in the elements. But I definitely think it's worth considering if you grill.