After defeating bowser and collecting 120 stars you need to go to the star island outside the star leap tower, then look up.
Hidden Palace Finale Edit
this is the final, and hardest sub-level in Super Mario Star Road, in this level you are required to do some of the hardest jumps in the game, when you get the star at the top you will unlock a replica stars in each sub-stage total of 9
get to the top of the palace one star is there along whit Yoshi and peach
Star Replica: Edit
The Star Replica is on a left platform Mario must first reach the top and then wall jump on the roof and then ground pound on the platform.