GameStop / Video Game Specialty Retailers
Your best bet to find a copy of Wii Sports is at your local video game specialty retailer. For many, that will be GameStop. The advantage of going to GameStop is that, in addition to having the largest selection of video games available at any retail store, you can also save money by buying your games used. Due to the fact that Wii Sports is a Wii launch title, picking this game up used may be the smartest option.
Online Retailers
Between online retailers like Amazon, Buy and NewEgg and auction sites like eBay, the only way you wouldn't be able to find and buy Wii Sports online is if the game didn't exist. While you will be paying a little more than you would by picking the game up at a brick-and-mortar store, you have the advantage of having the game being delivered straight to your doorstep. You may even save money if you're lucky and find an auction on eBay.
Importers
If you have a modified console or a system not native to North America, you can find some video game import retailers selling stand-alone copies of Wii Sports. The prices tend to be slightly higher than typical retail games due to the extra cost of importing the game and, unless you either own a Wii from the region the game hails from or you have a "mod chip" in your Wii, playing the game will be impossible. However, if you are a hardcore collector and have no issue with picking up a Japanese Wii, import stores are an excellent option.