* The International Music Score Library Project (IMSLP) is a non-profit organization that provides free access to public domain sheet music. IMSLP has a large collection of trombone music, including Boomer Sooner.
* The Trombone Page is a website dedicated to the trombone. The Trombone Page has a section devoted to free sheet music, including Boomer Sooner.
* Sheet Music Plus is a commercial website that sells sheet music, but also offers a selection of free sheet music, including Boomer Sooner.
You can also find free sheet music for Boomer Sooner by searching Google or other search engines. Just type in "Boomer Sooner trombone sheet music" and you will find a number of links to websites that offer free downloads.