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Really, any cable with power on one end and a battery connector on the other end will do. I got a cheap one off eBay that supported the 4 through the X, and had two alligator clips.
If you want to splurge a little on your power supply, get a cable like this, that includes the battery data cables. This will stop the phone from rebooting every few minutes while powered by DCPS. The cable also includes a probe so you don't have to boot up the phone with the power button or tweezers
If every penny you spend on this is a penny that you can't use to buy phones off eBay (believe me, I know) - and you have broken batteries - you can detach the control board from the battery cell and solder wires onto it. It will even have battery data!