The source repository for weinre is maintained at GitHub.
To build the current version of weinre, you will probably want to use Eclipse. If that's not an option, there is some experimental support in the main ant build script to compile the Java code outside of Eclipse. Explore the build.xml file to see how that's done.
At development time, weinre consists of 5 Eclipse projects. See the file README.md in the weinre.build project for more instructions on how to perform a build.