We tested out a bunch of USB Ethernet adapters for the Nintendo Switch and these are the 3 we have found that work (as of now). Nintendo did release an official ethernet adapter but it’s kind of expensive compared to these three USB 3.0 Network adapters that will work with the Nintendo Switch.
Plugable USB 3.0 Ethernet Adapter – http://amzn.to/2nXOfOX
UGreen USB 3.0 Ethernet Adapter – http://amzn.to/2nDmhqW
Plugaable USB Type-C Ethernet Adapter – http://amzn.to/2nD6c4q