I've noticed my local Best Buy having them the last few times I've been in. I'm in Oklahoma and Sunday morning seems to be the best time @ Target, Circuit City and Toys R Us. Best Buy and Game Stop seem to be on a mid week kind of schedule?, they seem more apt to have them in the middle of the week.
The sales clerk @ Target told me that if they advertise in Sunday's paper, they usually have them, but they go fast. Especially the Wii Fits, the Wii's themselves last a little longer.