Isn't that size a regular offering (at least in Europe)?
9 X 12 cm is listed in the standard Ilford product list as a 'stock' item. I've never seen it stocked by any of the British dealers, but it isn't really a UK size, being metric. Maybe in other European countries. I would have thought any dealer should be able to order it, though? It is listed as available "WW" (worldwide...) Limiting factor for those across the pond seems to be shipping costs, however.