Small Power Options For Parallax Continuous Servo?

I purchased a Seeed Studio Lipo Charger with a 3.7v battery, and am realizing the Parallax is most effective at 5v for my application. Can anyone recommend any compact battery options that will work? Here's what I have:

Also, do you happen to know if the LiPo Rider can still operate a let's say a 7.4v battery down to a 5v application then?

Thanks!

-Mario