Guitar Hero III

There are 5 total characters to buy from the shop:

Tom Morello ($10,000)
Slash ($10,000)
Lou ($15,000)
Grim Reeper ($10,000)
God of Rock ($20,000)

You will need to buy all five of them to unlock the achievement. At first, only two will be available. Beat the game and the three bosses will be available for purchase. They will cost a pretty penny though, $65,000. One play through medium and five stars on everything will bank you about $45,000. Just play and buy them when you feel that it's time.