Radio Shack should have 500 ohm resistors. I would recommend trying out different values and decide what works best on your system. Due to difference in internal wiring of the tonearm, listener preference, rest of the system balance, etc, not every system is same. Also keep in mind that lower the resistor loading, lower the output will be.
I am using Caddock resistors which I purchase seperately. I would recomend using either Vishay(little costly) or Caddock resistors, significantly better than stock resistor kit.
In case you feel that you need more gain, check with your preamp manufacture. For ARC gears, ARC provides simple modification(adding resistors) to change the gain up by 6dB. For example, most of the ARC preamp's 12dB single ended output can be changed to 18dB. If you are using balanced output, most of the ARC gears have 18dB w/o modification. If your preamp has less than 12dB, it would make more sense to either get a step up transformer.