Where can I buy sheet music for Rihanna's Umbrella?
You can buy from Sheet Music Plus or Music Notes.com.
You can find lyrics and guitar music for any song you want if you just google something such as "lyrics and sheet music for__________." But if you want sheet music for piano and other instruments, you will usually have to buy them. If you do find a website that will give you the sheet music, it is most likely illegal.