Build your own, Robin?:
http://usbip.sourceforge.net/ 
I have been meaning to try this on one of the devices we make at work (which have various combinations of USB, serial, Ethernet, WiFi and cellular interfaces).
There are quite a few off-the-shelf devices that claim to do this too although I have never tried any of them. Google for "USB over IP" and you'll find a bunch of them.
Cheers,
Rick.