12A current from 24V power source, are there resistors which can handle the wattage?
Is there a potentiometer which can aswell?


That's called a rheostat (basically a power potentiometer for the purpose you described) -- but I'd recommend against it, since they're pricey.


  If no, what would be the right way to go about this? Buy a ~300 W motor instead?


Easy -- use pulse-width modulation by using a MOSFET (and freewheeling diode) to apply 24V to the motor some fraction D of the time. Vary D and you will vary voltage to the motor. (Just watch the motor current.)

The MOSFET should be rated at least 40V and a low-enough on-resistance that you don't have to worry about overheating at your maximum current. Here's a BUK9509-40B which is rated at 40V 75A 9milliohm max at 5V Vgs.

(For some more info about the basics of driving MOSFETs from microcontrollers, read this blog entry of mine.)