The White Room
Use Luigi. Go to the floor where Tall Tall Mountain and the Water Bug Picture is.
EXTRA STAR!: Now, look for all the Toads on the floor without looking in doors. Talk to him. If you don't get a star look for another Toad. One of them will give you a free star after you talk to him! Lucky you!
Anyway...look for the room where you got Wario. There will be Power Flowers there. Get one and walk through the mirror. Go and open the door on the other side of the mirror. You should enter and see a White Room and find a star.
I hope this helps! :)