For which version of the JVM are you trying to find a debug build? If it's for JDK-1.6.0, you are in luck, since the jdk6.dev.java.net project has been making those available at http://download.java.net/jdk6/binaries/.
Don't forget to read the note on that page about installing the debug jar files: the installer forgets to make the executables executables (and might also forget that some things that should be symbolic links, on Solaris or Linux). I guess the feeling is that if you are willing to deal with the debug builds you understand the note, or can at least follow the directions.