How can I find the text of Death and the King's Horsemen online?
I'm afraid you're not going to be able to read the text online free of charge. Copyrighted texts are protected by law for the author's lifetime plus 75 years. To make such texts available for free access to all would infringe on the author's rights and be punishable by law.
You can purchase a printed book or a downloadable etext from either Barnes and Noble (bn.com) or Amazon (amazon.com) among other resources. Alibris offers previously used books at discount prices (alibris.com). You can purchase Derek Walcott's works there as well.