you can get servos at any local hobby shops, but better yet ebay. the HS-625MG is a metal geared servo, it lasts longer and the gears do not strip as easily. I have one. You can also look into the JR Z590m (also metal geared).
the thing you will have to look into though are the connector types, Hitec and JR uses different spline count on their servos, so you can't interchange the servo arms or servo saver.
the steering servo arm on my redcat can be used with Traxxas servos only.
here's an excellent resource on servos from misbehavin, part 2 has information about the spline count: