Well there's a s@3! ton if altezza in Northern Ireland and I know there's a few breakers over there (I usually see them on eBay) so that could be a thought to try get a standard one.
But to be honest I haven't really looked into cats yet but my plan with mine was to try and find a smallish performance cat cut up my exhaust and weld flanges in it so that it could be removable so id only use it for mot's. But even for something like that ut wouldn't need to be a performance cat you could probably use what ever you can get you hands on.
Sorry for not being overly helpful but it may give you some ideas on how to sort yours
Out of interest has you tezz failed an mot? Or do you just know the cat has failed?