You just need to get a few 12v to 5v voltage converters so you can get some more 5v amperage from your 12v lines. I use 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 you can see how I wire them up here: Scalable DIY SATA JBOD - Will store over 250 Disks! - #34 by enderTown