Cinco de Slayo can be purchased at the shop at any time from the main menu. Press LB until the shop menu is pulled up and select Cinco de Slayo. It may take a couple seconds to pop, but it'll show up shortly after purchase.
Coins can be earned through playing scenes or by playing Faces of Killed. I would recommend playing Faces of Killed as it tends to give the highest amount of coins in the shortest amount of time