iRobot Create 12-AA cell battery pack
I recommend not using the green 12-AA battery pack it ships with. Get a standard yellow Roomba battery pack and a standard Roomba charger to go with it.
If you're going to use the 12 AA battery holder, use high capacity AA cells. Something with at least 2000 mAh. The Create is not a walkman, it draws a lot of power. The Create manual suggests using alkaline batteries, but that's pretty wasteful because they're not rechargeable. The standard yellow pack uses 12 3000mAh subC NiMH cells, which means you should be able to use NiMh AA cells (like the ones from SparkFun). You shouldn't use the Create to charge them though, because the green battery pack is missing the temperature sensor lead needed to properly charge the pack. Instead get a separate AA charger like this one.
Of course, at $2 per AA cell and $20 for a charger, you still need to invest $2*12 + $20 = $44 more in the Create to get it running in a sustainable way. It is cheaper than a Roomba yellow battery pack ($60) and its accompanying charger ($40) but it's much more of a hassle to shuffle around AAs.