Anyone know about Band Hero?

My kids got Band Hero for the Wii as an early Christmas gift from my DS. It come with a guitar, drums and microphone, but I think you can play with 4 players. I'm assuming the 4th instrument would be a bass because when the guitar is plugged in, it asks if you want to be bass or guitar. So my question is, do they sell guitars/bass separately from the game?

I have looked, but all I see is just the game or the game with the instruments we already have.

We are very new to video game systems so I have no clue about this stuff. Also, what are good SR merchants to use for this type of stuff? I would love to get cash back if I could.