You can often find these items as part of a high quality Joker costume set, avengers costumes for adults but if you can’t find the right one you can always purchase a grey trench coat at a thrift shop and use purple fabric dye to get the right color. You can find a replica Joker cane on Amazon. To really set your Joker costume apart make sure to bring along some accessories like a joker playing card or his cane.
