In-game Shop: Pants and Skirts -> Pants
Price: 300 FC single / 375 FC account wide
These look to be a recoloured version of the pants in the pre-order bonus Snakeskin and Snakeskin, Tan outfits. You can combine these pants with the Open heavy stylised jacket for pretty much the same look as the uniform.
Note: The black version of these pants is labeled as “Pirate motif jeans” instead of “Skull and Crossbones Jeans.”