You might go to www.towerhobbies.com and under Radio acc. type "high torque servos". you don't have to buy from them but it will give you a list of what's available. I wouldn't choose anything less than 125 in-oz. at 6v. Personally I use Airtronics 94357 but they're at the minimum torque rate and I'm a minority. I think most use Hi-Tec or Futaba.
I use Futaba S3305 for steering, they are about $35 (each) and I prefer the Hitec HS-5745 (about $70) for throttle. The throttle servo does a ton of work and gets punished the most, the 5745 has a heat sink around it and that helps keep it cool on those long wide open runs.