There used to be a device that plugs into a computer's USB to act like a wireless hotspot for DS and Wii, but that was discoutinued in 2007 or so. If you can still find one of those with drivers, it may work. It will use your computer's dial-up access as a source for the wifi signal that would get picked up by the Wii.
No idea how easy it is to find one of those now.
Is there no other way to get broadband to your home, where you can then use a cheap wireless router to connect to the broadband modem via ethernet to give yourself wifi in the home?