Players can purchase the character from November 23 to November 26 on the popular Facebook game.
Originally available as a gift to players that obtained 15 mastery stars in Spec Ops 1, Mockingbird will be accessible to players again. Marvel has not revealed how much the character will cost. There is a Wikia entry on the character that should give those that are interested in obtaining her should check out.
Marvel: Avengers Alliance can be found here.