How long to get a PS3 or Wii? Well, depends on how far away the nearest person who owns one is, how long it takes them to fall asleep or go on vacation, how quickly you can pick a lock...
Oh, wait, misread the topic title. You want to
buy one from a
store. Sorry, that was just how I get most of my consoles.
Both Nintendo and Sony are going to be shipping more units before Christmas, but those will likely be sold very quickly as well. I looked over GameSpot, but didn't see any articles on that. Your best bet may be to ask whatever store you go to when their next shipment is coming in.
EDIT: Okay,
this article indicates Nintendo will ship four million Wiis worldwide by year's end.
this one from GameSpot may also help out. Can't find anything real solid, though.
As for when they'll be easily available like the PS2, GCN, and 360 are now, I'd say a couple weeks after Christmas.