A wireless modem might work. You cannot connect it directly to the Wii to get internet. You would need to make sure it is hooked up to your computer and internet access. Locate your SSID or access point name. Then click Search for Access Point on the Wireless connection area of the Wii under Wii settings. Then click on your access point name. If it is locked, you will need your WEP key or password to enter in. Then you just test the connection and see if it works.
Another way is to get the Wii LAN Adapter for wired connections. They cost about $30. You would plug this into a USB port behind the Wii, plug an ethernet cord into the LAN adapter, then plug the other end into your modem. Then you would choose Wired Connection and simply test it.
Hope this helps.