For initial experiments, I searched around my house for an old wallwart in the right range - 6-7 volts IIRC. The ones I found didn't provide enough current for more than a couple of servos though. You may need to buy (or build) something that provides more juice. Even though your robot sounds like it will be stationary, you might look at lipo batteries to avoid the need for acquiring what would effectively be a beefier wallwart.