They're hard to find because they've been pulled from the shelves!
Read up on the news. Guitar Hero 3 for the Wii has a bug where the audio outputs in MONO... not stereo or Dolby Pro Logic II like its advertised... MONO.
Activision instructed stores to pull them from the shelves.
If your looking for it, return it if you can. The fixed versions are supposed to be released closer to Christmas (if they can make that deadline).