All iPads come with a lightening to USB cable so you can plug it into your computer.
Perhaps you could be more specific in what it is you need?
I suspect you want to plug in a standard USB memory stick.
You might want to consider bluetooth solutions instead.
There are a number of drives that connect to the iPad via bluetooth.
http://www.laptopmag.com/review/wireless/seagate-wireless-plus.aspx
This is kinda what I'm looking for but the USB side needs to be "full USB side" - not Mini or Micro.