I'm building a charging station in our bus and just went through the internet search for charging USB ports. Just a few things I learned:
Apple devices like Ipod, Ipads and Iphones require special wiring in the USB port for charging. So be sure if you buy a USB port be sure it specifically indicates it will work with Apple. This is especially true for the Ipad which will not usually charge fully on a standard laptop or computer USB port. That's why the Ipad comes with a 120v AC adapter and special USB cord.
You can buy multi-port USB adapters, but be sure they are 'charging' ports not just data ports.
Most USB ports only support 5 volts DC and 1 amp of current. An Ipad requires 2.1 amps and finding a charging port that supports 2.1 amps is not easy. I found some hard-wire devices here: http://www.12voltdesigns.com/Products/USB_Charge_Cable_USBCC2.php
I bought six of these as they work with Apple devices (w/o computer connection) and so will charge any USB type device.