I have a wifi only Acer chromebook. I also wanted an option of a wired connection when I go visit my sister who doesn't have a wifi router. I bought the "Trendnet TU2-ET 100" USB to 10/100Mbps Adapter and it works, I am using it now as I type this message. It works right out of the box, you can ignore the included setup DVD. I got it from Amazon for $17.69.
Google's help on this says a wired (USB) connection take priority over wireless and it seems to work that way. Instead of wifi bars in the upper right corner of the screen I have an icon that looks like an Ethernet port.