I didn't find one until I got my lazy ass into a Circuit City before noon on a Tuesday.
I found a Wii the same way I found my 360. Every damn place I went I looked and asked. If you do that then sooner or later you will get your Sister one.
And if you are thinking about asking people when they get stock in and stuff don't forget that getting the stock in is useless until they have unpacked it. If some Wal-Mart guy says 10am every Tuesday you may want to think about showing up at 11am.