You just need a few of these: Amazon.com: BANKEE DC 12V 24V to 5V 15A Converter DC Voltage Reducer Regulator Step Down Buck Converter Power Supply Volt Transformer Module (12V/24V to 5V 15A) : Electronics
And here’s how I wired them up to the SATA power cable: Scalable DIY SATA JBOD - Will store over 250 Disks! - #34 by enderTown
I can run 12 disks off of one of these with no issues!