Killing Pirates

Or joining them

I joint the Crimson fleet but I didnt get the suit

You can buy it from the armour vendor on the key.

Get lucky with random loot drops. Trust I've been there.

General rule if you want something:

Find someone who has it and kill him for the next two hours:)

Obviously, it's been answered. However, You can receive this by either finding it off dead pirates, finding it on random loot drops, or of course purchasing it at a vendor.