MOST hobby shops carry throttle return springs. Team Losi makes a good one and its about 3 bucks. You could use a spring ment for a exhaust manifold too, you just hook one side to the chassis and the other to either the horn or the carb. Depending on the rules, you will have to mount it to the carb if it is a ROAR rule or the opposite side of the horn (if its on the horn it could be bad if the carb linkage pops off). Exhaust springs might be too much tension, so I just get the Losi one.
Good thing they make you use throttle return, I've seen too many runaways and its usually because they don't require it.
Detroit Radio Control Car Club