Just make one out of expanded sheet metal. You have one all ready so you have a template to work from. IMO This is as cheap as you can get one. I use these for cheaper than buying one.
I got some Grill Mark 26" grates from Ace when they were closing them out. First place I used the Google machine came up with this outfit-you can't beat the price! Don't know what shipping will be though?
Actual size is 24 3/4" dia. But I have a couple old style one piece grates from Weber that are the same dia. The Weber stainless steel grates with flip up sides for the 26.75" OTG are 25 3/8" dia. According to my Stanley tape measure!