If you must locate the paint bucket look in the Tools palette and find it beneath the gradient tool. Alternatively, use the G key shortcut (or Shift+G depending on your preferences) to toggle between the gradient and paint bucket tools.
For quick use just press Alt+Delete (or Alt+Backspace) for a foreground colour fill, and Ctrl+Delete (or Ctrl+Backspace) for a background colour fill. The paint bucket tool, however, provides more options.