Each week, a user named speedy1961 posts early info about the Target, Best Buy, and Circuit City ads in the following forum:
http://forum.dvdtalk.com/forumdisplay.php?f=1
If you check there toward the end of a week, you'll know which, if any, of those stores will have Wiis in stock the coming Sunday morning.
BTW, looks like Target and Circuity City will both have Wiis this coming Sunday.