Finding the System
Chances are that you won't find the Wii on "sale" in the normal sense of the word. While the cost of the system will drop over time, finding it for a certain percentage off just won't happen. However, going to a store that specializes in video games, such as GameStop, will give you an alternative to paying full price for the system.
Used or Refurbished?
Most video game stores will have a stock of refurbished Wiis. Refurbished Wiis are ones that have been used but have been sent back to Nintendo for repair. These systems are usually in perfect condition as any issues have been fixed. If they are selling a "used" system that has not been refurbished, then that means it didn't go anywhere for repair. Often an employee will have checked to see if it still works by turning it on.
Checking the System
After deciding on a used or refurbished system, the store employee should hook the system up for you to show that it works. If he doesn't offer, ask. If for some reason he doesn't want to hook it up (and he doesn't accept refunds), chances are you shouldn't buy it from him. Make sure everything works, that it can play games normally and that Wiimotes respond to the system. After checking out the system, ask the employee about the store's return/exchange policy just in case you find a problem with it later on.