Take 7.5V from 12V battery
Probably you can benefit from an off-the-shelf DC/DC step-down converter module. They are compact, they have good efficiency and they are hassle-free, i.e. you don't have to design all the circuitry needed to build one from the ground up using an IC and reading its datasheet to get all the parameters right. There are some models which are really cheap, too!

Off the top of my head, see this Pololu page.

As @BattleHamster said, you need to select one knowing what's the maximum current your servos will be drawing (with some headroom for safety).