Sunday mornings are your best bet. Most major department and drug stores (not counting vg-specific places like EBgames, Gamespot, etc; they might run on their own schedule) in the US run weekly sales ads that start on Sunday and end on Saturday. So when stores get new systems, and said system is going to be advertised in the following week's ad (though that very rarely means they're actually on sale), that means the store's not going to sell the product until they open on Sunday.
Therefore, you need to start calling all the department stores near you on a daily basis. Wal-mart, Target, K-mart, etc. When you find one that's getting them in, find out what day they're going to start selling them (if it's not Sunday), find out what time they open that day, and be waiting outside the store's entrance an hour before they open that morning.
This tactic, if you live in a relatively high speed location (even small cities get systems often), will get you a Wii in two or three weeks at the most. If you don't force yourself to call stores daily in your search, then you won't find a system for some time. You're going to have to be persistent if you expect to find one.
For the record, I used that tactic, and found a Wii in three days from when I started searching. But I do live just north of DC, in an area of Maryland that's pretty busy, which would theoretically get systems more often than smaller cities. But still; three weeks or less. That's all it'll take if you get busy.