I'm not sure if USB 2.0 can handle Gigabit Ethernet so there may not be any adapters. However, I may be wrong.
Just did some research and found a Belkin USB 2 Gigabit Adapter...
http://www.belkin.com/IWCatProductPage.process?Product_Id=281799
But it also has this warning...
*NOTE: The stated transmission rate-1000Mbps-is the physical data rate. The actual USB 2.0 throughput will be lower than the physical data rate and is dependent upon the PC's USB capacity and other data traffi c on the USB bus. In addition, throughput results will vary depending on the PC hardware and OS used.
So it sounds like it might not work for full gigabit speed, but it would probably be faster than the Apple one.