Well you can either buy that (i'm sure you could find it for like $80), or if you have like a laptop you can connect that to wifi and then plug your ethernet cable into the laptop and xbox and then on the laptop create a network bridge between the network and the xbox. At least that's what I'm doing now.
A better option in terms of connectivity would be to plug your Xbox into the modem, and then get a wireless adapter for your computer. They usually run about $35-50. Make sure you get an internal one, not a USB one. USB are more expensive and overall WORSE.
A better option in terms of connectivity would be to plug your Xbox into the modem, and then get a wireless adapter for your computer. They usually run about $35-50. Make sure you get an internal one, not a USB one. USB are more expensive and overall WORSE.