You....build a motor driver.
A motor driver is nothing more than a current amplifier. It takes a small current (from the Arduino) and allows a larger current to flow in response. The simplest motor driver is just a transistor. A small current into the base of a (BJT) transistor allows a larger current to flow into the collector. If you just want your motor to go in one direction then this is an easy way to do it.
The motor's inertia smooths out the on/off behavior of the PWM signal to give a reasonably-constant average speed that is proportional to your PWM duty cycle.
If you want your motor to go in both directions (forwards/reverse) you will need to build an H-bridge (4 transistors).
Originally you stated "I have a power source that takes AA batteries that is meant for some dc motors." If you can tell use how many AA batteries (i.e., the voltage of the battery pack), how many motors, the voltage/current rating of the motors, etc. then we may be able to make some more concrete suggestions.
The Aussie Shield
: breakout all 28 pins to quick-connect terminals