While Apple does sell a USB ethernet adapter, it's limited to USB 2.0 and 100 Mbps connections. Apple's recommended solution for gigbit speeds is the Thunderbolt to Ethernet adapter. Depending on your desk setup, you might have more free USB ports than you do Thunderbolt so it becomes a difficult trade-off.

AmazonBasics sells a USB 3.0 gigabit ethernet adaptor for only $13.99. This allows you to have the best of both worlds: gigabit ethernet speeds while not tying up a Thunderbolt port. You'll want to grab the driver download to make it work.