
On the Dean & Nooch podcast, Nintendo president Reggie Fils-Aime promised that there will be plenty of Wiis in the stores this holiday. Not that he thinks it's really going to make it any easier for you:
Reggie suggests you regularly check your local Toys R Us, Best Buy, Circuit City, Wal-Mart, Target, or GameStop to see if they've gotten a shipment in and "try and get there when the product comes in." In other words, what you've already been doing. Hopefully the increased numbers of Wiis coming in will increase the odds of snagging one, but plan appropriately.
Reggie is actually more concerned with stores having enough shelf space to accommodate the vast number of Wii and DS games that are coming out this holiday season.
Interview with Nintendo's Reggie Fils-Aime [Dean & Nooch on Gaming]