there's lots of places online to get ESC's...
If you want to drive a heavier load servo, just make sure you follow the BEC (battery eliminating circuit) specifications. I THINK you just need to connect the ground wire of the cable from the pic to the ground of the battery you use together (reference same ground), signal wire goes to servo, and the positive lead from the battery goes to the positive lead of the servo, thus not using the vex to power the servo. JUST MAKE SURE that the battery you use is NOT more than the input voltage of whatever you are driving. There are also optocouplers that you could use to drive the servo off if you want.
I would just be safe and get a cheap ESC, i have one if you want it, send me $10 and shipping costs and its yours. Its a novak.
here's a good site too, for drivers:
http://www.pololu.com/products/pololu/#servcon