A power MOSFET would probably be easiest to drive. Use one rated for 3A or greater. They typically take 8-10V or so for best turn-on, so operate the 555 at a 12V supply or greater (15V max.).
What is you PWM frequency?
A search at Maplin came up with an IRFZ24N, IRF740, and IRF540, somewhat overkill for you requirements, but any one should work.
Make sure you place a suppression diode across the motor (cathode to plus motor connection) to suppress inductive spike voltages that can blow the transistor.