You looking for a Male-B to a Female-B, extension cable.
They aren't exceedingly common since the USB spec doesn't approve of them. Keep in mind that USB is a differential serial bus and not the simplest of cables to make properly.
Which arduino do you have? Mine isn't a "mini" B, it's just a B.
Just go on to somewhere like Element14 / Newark / Farnell (whatever they're called in your country), or mouser, or digikey, and type "USB Panel" into the search box - see what you get.
I'm not sure exactly which type you want, but I did a quick google for USB panel mount, and this site was one of the first that came up that offers different ends. They don't seem to offer mini and micro USB connectors for panel mount however: http://www.cablestogo.com/product_list.asp?cat_id=1532.
Make your own cable. As I mentioned before, just get two cables, each one has one end that is right. Chop them up, and reconnect them in the combination you want.
For a more professional finish, buy a panel mount socket, a right-angled USB mini B plug, and a length of wire. Build your own cable from scratch.
This, that annoying clock with the buttons and output pins, understanding the code and the like, I am really taking it slow so as to not stuff things up.