If you don't need the entire game, perhaps you are just looking for a spare set of drums? Good news! The drums are also on sale.
Of course it wouldn't be news if it didn't have co-op, and Rock Band is king of that, with a 4 player co-op band mode!
And while the Amazon Rock Band Store isn't as sexy as The Official One, becoming a Rock Band owner via it will let you feast your eyes on the official one's face melting goodness.