I've heard great things about the Perfect Pass Monster 56KG. It's a buy-once-cry-once option. It can also take up to 8.4V for even more power and speed if you decide to change your other electronics in the future.

Also, you're going to need a servo extension wire with most servos but don't let that stop you. They're literally just a plug-and-play extension wire. Here's a cheap 5 pack you can purchase right on Amazon with the servo. No fuss at all.

Here's a nearly identical question from yesterday if you want more suggestions.