I use Kadee G scale #878 (they are long, just cut off a piece as long as you need), or Kato GS-4 pilot truck springs.
It really depends on the diameter, stiffness, and length you are looking for. But those two cover a lot of cases.
Also, some Bachmann coupler box springs from old equipment work well. I always save springs of every type from old model train stuff I come across, no matter what the scale. You never know when one will be just right for a truck spring.
