Randomly going to a store during the week is likely to be a bust, but have you tried going to a store on Sunday morning?
I’m speaking from the Baltimore-Washington I-95 corridor here, and know that it is routinely not difficult to go first thing on a Sunday to a Circuit City (where they’ve had a voucher program going since the Wii came out) and Best Buy (where it’s just a good bet, and where I got one from the Arundel Mills location), and be able to buy one.
I learned from a Best Buy manager that they will hold some stock of Wiis in the stock room for Sunday morning, when there’s an added demand in “hey maybe I’ll buy something else while I’m here” courtesy of the new week’s circulars, and I’d assume that for Circuit City to hold stock for vouchers every Sunday (usually the small print in the circulars will say something along the lines of “minimum 10 per store” that would hint at some hoarding).
I’ve also noticed that some stores like Walmart or Toys’R’Us, which are not the first thought for people trying to get video games, are more likely to have stock on any given day than a place like Best Buy.
ETA: A friend had success getting a Wii from a Target in a sketchier area, under the assumption that the people in a lower-income area are less likely to jump at geeky new video game systems. Pair with the Sunday morning guess, and you can probably grab one.