RCBS makes a Hollow Base mould that duplicates the .455Mk II 265gr. bullet. It's a little pricey at around $120.00. Midway has them listed.
Another choice would be to buy the same bullets from Buffalo Arms at around $33.00 per 50. (Just how much you plan on shooting would be the decision maker..) They are back ordered right now and expect to restock in April.

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Dean
SWCA #680 SWHF #446