Henry,
Post or search on USA7s(friendly enthusiasts)
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Ben at Caterham USA
Caterham Cars has these:
https://www.caterhamparts.co.uk/product_info.php?cPath=105&products_id=124&osCsid=efb6690d008d73af550d7da8566275d9
They do have or had various sizes. Very expensive but the grip is amazing....
For my sins, I love Elans.
Ed