Go to Google Images, search for the book cover.
Right-click on it, choose save image as
Save to the desktop
Click on the book in Calibre
Open the 'edit meta information' area by clicking on the button
Go to the bottom right corner where it says 'change cover image'
Click on the left button underneath it (the one beside the garbage can)
Browse to where you saved the image
When you click okay and exit the meta information editor, you will have a cover