I find that Wii Fit are hard to come by. For some reason the local retailers can't keep them in stock.
You'd think that they'd make plenty of them, considering it's the only video game that actually gives you exercise!
At any rate, I've found a place that always makes sure that there are units available:
Wii Fit For Sale
Check it out!
Wii Fit For Sale
You will also want to get the Wii Fit Tutorial! This guide tells all the secrets: hidden jogging trails, unlock codes, even cheats! In addition, it includes a workout routine (surprising NOT included in the Wii Fit itself!). So go download the Wii Fit Tutorial.