I think they are all alloy, but I also thing they vary from car to car, so I think you'll need to say which Jaguar you are trying to find a bell housing for.
I didn't want to post a link in case I get told off for trying to sell it on here, seems you don't accept emails, if you search ebay 'Jaguar E type Mk2 Moss box bell housing aluminium' you should find it.