Specialist Contracts can be obtained from the Contract Box.
Rewards are going to be the Specilist Skin of the item and a Specialist Coin.
Once you got 10 of the Specialist Coins, you can go to Blue Ridge (Colorado) and exchange it for a Specialist Crate that contains rare loot. The mission is available on the map through the Missions tab.
The magazines gives you a stack between 3-5
Changes can be done any time