A short, finished list rather than an endless feed: 100 of the best sad quotes ever written, picked out of the full sad archive on QuoteForever. You will find Peter Steele, Sylvia Plath, Dietrich Bonhoeffer and Paulo Coelho among them.
Read the list top to bottom, or use it as a starting point - every author name links to everything else they said, and every quote has its own page you can share or turn into an image.