You can buy Wii Points from most online stores.
You can also earn stars from visiting the official Nintendo website -
http://www.nintendo-europe.com . There are 5 stars per game you purchase (code on a slip inside the game) and you get 1 star for every day you log in on the website. After a while these stars will entitle you to free stuff but it isn't cheap, I think it's something like 5000 stars for a free game, and 150 for a wallpaper etc.
If you're looking for old games like Sonic and such why not get an emulator? Try here :
http://www.coolrom.com
Although there are some people on eBay who are selling like 2,000 Flash games for your Wii but it's mainly games that you can find on game websites.