The largest capacity battery currently available that fits internal to the FT-817 (that I know of) is the Windcamp 3000 mah pack. Next is the W4RT pack at 2700 mah.
External batteries are available in a multitude of sizes. If you are going to go External, I would also recommend the Wulfden Power Pole adapter/regulator.
Is this the Wulfden power pole adapter/ regulator you are talking about?
http://www.wulfden.org/downloads/manuals/Condx817RevA1.pdf