These battery holders provide the right voltage for many kinds of Arduino based projects. The 2x18650 size is the default provided with the Vorpal Hexapod, the 6xAA and 6xAAA will also work but will not provide as much run time.
These holders are available both with bare wire ends and with female JST connectors.
NOTE: rechargeable batteries are required to provide enough current (amps) to power the Vorpal Hexapod and many other motor based projects, disposable batteries typically cannot provide enough current to run more than a few small motors. Some projects you might use these for may be fine with alkaline or other disposable battery technologies.