I've never actually bought a full-size one. Used to used whatever off-brand was cheap & never realized what a waste of money they are, until I kept getting free ones when I purchased used kettles. If memory serves, I did spring for one of the mini ones at the Weber Factory Store during one of my trips over there when it was still open.
@michaelmilitello Do you ever head up to the O'hare area? If you do, I'm sure I can scrounge up a FREE used one in good shape for you and probably even an 18 grate if you still need one.