You can buy items from the unlock shop using the style points you earn playing the single player campaign. It will cost you 7,650 points total to unlock everything.
For tips on farming style points, see the guides for the following achievements:
You will probably want to concentrate on purchasing the multiplayer characters first so that you also unlock the following achievement as soon as possible:
The Freedom! achievement in Stranglehold worth 28 pointsPurchase all of the Multiplayer characters in the Unlock Shop