The maven dependency plugin can potentially solve your problem.
If you have a pom with all your project dependencies specified, all you would need to do is run
mvn dependency:copy-dependencies
and you will find the target/dependencies folder filled with all the dependencies, including transitive.
Adding Gustavo’s answer from below: To download the dependency sources, you can use
mvn dependency:copy-dependencies -Dclassifier=sources
(via Apache Maven Dependency Plugin doc).