You could go to your local electronics store and see what they have. Most likely, they will have a transformer that plugs into household current and gives you the voltage and amperage you need. $10-15 I expect.
The ratings are the maximum voltage and current draws. You will want to use a lesser voltage to slow things down, and hopefully the motors draw much less than 500ma in operation so they stay cool.
An ordinary DC power pack is just fine, and will give some control over the speed of the motors.
