I use IDX 14V Lithiums. I have 2, and 1 batt. usually gets me through a 10 hour day running my 644, 2 Lectrosonic 411 receivers, and a phantom powered boom. You'll nee a cup to attach to the battery, and a power cable for the mixer. Check out Trew audio for all these parts, or Location Sound in LA.
The battery is about $200 and the cup and cable probably $150. (guestimating!) The charger for IDX is another expensive item. My set up includes a power distribution block with 6 outlets to power mixer, radio receivers, and whatever else I might need depending on the shoot. This set up seems to be pretty standard in my world of ENG bag work. Hope this helps.