asg, a return spring, is a spring that is hooked from carb slide to the chassis in such a way as to pull the carb hard shut if you lose signal, like an electronic fail safe only a mechanical version.
As to getting a failsafe, if you go to 2.4ghz (spektrum, futaba fasst etc) you prolly wouldn't need one. I have 2.4ghz on a revo and a thunder tiger eb4 S2, NEITHER of them have a failsafe and NEITHER have suffered signal loss or interference since switching over from 27MHz AM to 2.4ghz.
BTW asg, where you at ??