See if you can find an electrical wholesaler. If not Lowes/Home Depot would probably be your best bef for the 12/3 or 14/3. Also figure out which one you need. 12/3 can handle 20 amps of current where as 14/3 can handle 15 amps. (I'm pretty sure those figures are correct, if not please correct me.) Also don't forget the 10% rule. You only want to use 18 amps on a 20 amp breaker (10% of 20 amps is 2 amps..so subtract 2 amps from 20) and 13.5 on a 15 amp breaker. The reason is if your voltage is low you will pull more ampers to curve the low voltage. That way you can handle a low voltage without worrying about your wire.
The 10% rule is one electricians tend to follow, though I haven't heard much about it in the tech business but I figured I would share it.