There are a couple websites that will stream through "Opera" which is the browser on the Wii. You might want to google that. I usually just watch things that are on my computer or through youtube.
You could just use an S-Video from a laptop or computer as well if you're TV is set up for it.
I have a macmini hooked up to my tv. dvi to hdmi. If you have a computer near your tv, its pretty easy. As far as a wii, Im not sure. I dont use mine enough to know.