Unfortunately, physical copies of William Morris's works can be hard to find. I will try to list the best places to buy his works below. Dover currently has available on their website new paperback copies of The Wood Beyond the World, The Story of the Glittering Plain, The Sundering Flood, and News from Nowhere. There is also a Penguin edition of News from Nowhere and other writings here. Past that, ebay and abebooks can be good places to find used volumes by Morris.
Most of Morris's work can be found electronically at Project Gutengerg. If you are interested in buying physical books, most of Morris's work can be found in The Collected Works of William Morris. Hardback versions of his collected works haven't been printed for some time and are very expensive to buy used. Paperback versions are available through the Cambridge University Press as the 24 volume set or individually.
This set of collected works makes several notable omissions:
First the two volume set "William Morris: Artist, Writer, Socialist" by his daughter May Morris and George Bernard Shaw. Also available through the Cambridge University Press.
Second, the six volume "Saga Library" by William Morris and Eirikr Magnusson (see bibliography page for more details). Sadly the Saga Library is out of print and very rare; perhaps this should be changed.
For the true Morris enthusiast, there is also a very nice four volume set of his letters "The Collected Letters of William Morris" compiled by Norman Kelvin. These letters are available through the Princeton University Press and are not cheap by any means.