Phillip I think it's just a heatproof mat, if so they range from £1 (local pound shops) to £9.99 for a heavy duty one, so the postage would be negligible as they are only grams in weight. I have them for my fan oven base to aid clean up. Supposedly they can be cut to fit pans to aid in the non-stickability.
Having seen a picture of the trivet it looks similar to Lakeland's additional oven shelf, again versions available at vastly cheaper rates from catalogues and pound shops.
http://www.lakeland.co.uk/13106/Oven-Add-a-ShelfGranted it's not a Weber but would do the job. Tala also do a range of grids fro cooling cakes etc in various sizes, maybe one of those could sunstitute for the proper trivet? Or a visit to a charity shop for an old roasting tin and rack might be of value.