7.2V will work but may get less power on time ascameras are voltage sensitive.
7.4V obviously works since that's what internal batteries are rated at.
8.4V OK too,there is voltage regulator on DC input jack.
You can buy 3.7V Li-Ion cells in a variety of sizes/shapes (common and not so common) from places like www.megabatteries.com
or try a local hobby shop thats carries RC goodies.