I'm looking for a good onboard L.E.D. light for my GY-HM100.

I'll be shooting primarily indoors during the day. I would rather it provide its own battery source, but I'm open to advice there.

Any suggestions?

The Switronix TL-50 is probably one of your better options. Get a cheap flash mount and mount it onto the flash mount on the hm100. It uses lithium ion battery packs and it lasts at least two hours (it was at medium+ power at the 2 hour mark). Come with a wall charger to recharge. Light intensity can be adjusted via dial in the back, and has high,medium,low battery led indicators. I would buy the same thing again.

Amazon.com: Switronix TL-50 30w Dimmerable DC Powered LED Light Fixture 5600k: Camera & Photo