Book and music shops along with charity shops are good sources of organ music. Gumtree and Ebay tend to have listings as well as Amazon. For more specific purchases look online at some of the retailers listed below which between them offer a wide range of new and used music. There are often free copies of music available on some of the music groups mentioned in Other Groups and Organisations with composers sharing their latest offerings.
Dave Barton Second-hand Sheet Music
The Organ Manual 