How do you get infinite bullet bills on Mario kart wii?

If you soft-mod your wii (setting up the homebrew channel and/or other non-Nintendo licensed content) you will be able to add hexadecimals, commonly referred to as "gecko codes", to the loaded version of your wii game.

For instance, you could add hexadecimals to your Mario Kart Wii game (RMCP, RMCU and RMCJ are the game codes for Europe, America and Japanese versions respectively), which tell the game that all players always have infinite starmen.

The game will load this hexadecimal, thus it will be true.