In missiles, what actuators are used for the control surfaces?
Battery powered servo actuators.
From www.acc.af.mil, for the AIM-120 Scorpion:

The control section consists of control electronics, actuator batteries, and four independently controlled servo-actuators.

Further on the actual AIM-120 control mechanism:
(Courtesy of the Turkish AF http://web.deu.edu.tr/atiksu/ana44/air3.html)

The Weapons Control Unit (WCU)-11/B consists of four independently controlled electro-mechanical servo actuators, four lithium-aluminum batteries connected in parallel, and a steel fuselage section that is bolted to the propulsion section aft skirt. Each actuator consists of a brushless DC motor ballscrew, an infinite resolution potentiometer directly coupled to the output shaft, and pulse width modulated control electronics. The output shaft is engaged directly to a squib actuated lock so that it does not interfere with the fin (control surface) installation and removal.