If I can access the internet without a router, can I buy a router just for my DS and Wii?
Short answer: Yes. :)
EDIT: But wouldn't it be cheaper to buy the Wii/DS USB Port? But then again, with the router, you could connect other devices in the future if the need arises. It's your call.
1. Please use the Wi-fi discussion boards even though they are slow, your question will be answered eventually. :P
2. Yes, get a Wi-fi USB adapter. I believe it's about $30.