You may already have a platform in the works, but another possibility is 6V SLA batteries.
They will likely run a 9V motor ok too. You can find SLA batteries of many sizes, and can
pick the best size,
BTW, I have an Omnibot 5402 that uses the 6V, 4.5 AmpHr SLA, and runs for a long time,
http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_264014_-1
[beware the $1250 and $900 auctions! I got mine for < $20]
I've also run the Omnibot off a set of 6 NiMH AA Cells, which go 2500 mA-hr these days,
and that works good too.