You can find poetry online through the database, Literature Online.
Books of poetry are organized as part of literature as a whole, which is arranged first by country, then by author. So everything by W.B.Yeats is together, within the area of British literature, whose call numbers all begin PR.
If you want to listen to poetry read aloud by the poet, you can browse our Voice of the Poet audio collection beginning at Audioset 383, tucked between the DVDs and VHS videos.
Recommended websites: http://www.poetryfoundation.org and http://www.poets.org.