I've bought one of these:
Duble USB Output Power Adapter 12V To 5V 3A 15W DC DC Converter Module Connector
very pleased.I particularly wanted 2A outputs that worked over a wide range of input voltages.

It's surprising how poor the power density of AA batteries is compared with modern phone or laptop batteries. So I'd recommend an external USB battery pack, which is very versatile and easy to charge up on the move.

Well, Bestek is the best brand for many powerful inverters, these inverters are generally car power inverter but you can use somewhere else depending on your requirements. Because all the batteries are of 12V, I think.