Disclaimer: this is incomplete and work in progress, please report problems.
People normally don't need to directly hack the stylesheets sources.
You can find official releases at the DocBook Project file-release area at SourceForge
- Numerous Linux distributions also provide docbook-xsl packages that you can install using their package-management systems
So-called "snapshots" are automatically built each time a change is checked in; the snapshot builds are available from http://snapshots.docbook.org
If you need to customize them, simply check DocBookXslStylesheetDocs
For instructions, see README.BUILD.