As you suspect LiPo would be your best alternative, though it's not without its drawbacks. Quite apart from its tendencies to explode & burn your house down at odd moments LiPo requires special chargers that can provide a balanced charge (which aren't cheap). You're probably not going to need (or even want) higher voltage than the 9.6V - chances are the ESC is only rated that high. Higher mAh is what you're after for longer flight times, though be careful of adding extra weight (again why LiPo's are better, they're much lighter and have much higher current capabilities).

If you're not prepared to shell out for the LiPo charger, I'd look at building my own packs. Make sure you get the cells with the solder tags already in place as soldering directly to a cell will heat up up so much you'll probably damage it. First thing I'd do though (before shelling out any cash) is learn to fly your plane and see what the standard battery's capable of. There's no point in spending big bucks till you know you're not going to kill the plane, lose interest, etc.