You'll be able to find a Wii pretty easily -- you may need to go to a few places on launch day, but I wouldn't worry about acquiring one. And I'm absolutely convinced Ninty will be restocking shelves quickly and without the same problems that plagued the 360 and now the PS3.
You have to remember the Wii is NOT technologically difficult to manufacture. I'm not dissing the system, but it has no dual cores, no CELL, no Blu-ray, etc. This is, as many developers have stated, Gamecube on steroids. Even IBM went on record to talk about how "reliable" the Hollywood/Broadway chips are -- that's industry-speak for, "this ain't cutting-edge, folks, but good enough for what Nintendo wants to do".
Nintendo promises 4MM units by end of 2006, and they should have NO problem filling that. They've been producing these consoles for a while now, and with many fence-sitters such as myself who REFUSE to pay $300 for Zelda and a souped up Gamecube (there are quite a few of us, BTW), and will wait til the inevitable drop to at least $149-$199, that should leave more units for some of you who are dying to pick it up ;-)
Now, if casuals truly latch on like they did with the DS, then you may need to do a little more snooping and visit a few more stores, but you'll find one. I for one don't think the Wii will strike it with casuals right out of the gate -- it will take, as it did the DS, a few months to do it, but the Wii should be the DS-console equivalent in time.