You can get any battery pack you want, and put it where-ever you want...It is your truck after all[8D].

Check the plug type on whatever pack you are looking at and make sure it will work in your Rx.
I have used a couple of different plugged servos and batteries, and used them together...Some require a bit of modification with a sharp x-acto blade, Or I guess you could replace the end with the correct one easily enough, too.

As far as the location...Keep in mind that lower is better. The battery pack is a fairly heavy component on the chassis, and it's placement will have a dramatic effect on your trucks handling. I moved mine in my tub style RC10GT up in front of the throttle servo, and relocated the Rx.