Main source code at Eclipse
The main source code is hosted in GIT repositories located under the Eclipse Foundation Github organization. All of the repositories are prefixed with microprofile.
The main repository is the eclipse/microprofile repository which contains information for newcomers and some shared resources. However, the information on this wiki tends to be more often updated.
The repositories within the Eclipse Foundation are also listed in the MicroProfile project's Developer Resources
Note: If you want to contribute to these repositories, follow the guide in Guidelines for contributing.
Additional source code and resources
Additional resources can be found in the MicroProfile GitHub organization. These resources are mainly related to the microprofile.io website or other activity not related to the MicroProfile project's deliverables.
Source code and resources from this location are being slowly migrated to the Eclipse Foundation where it makes sense.
Note: For contributing to these repositories, just raise a pull request against a repository. You should license your resources under the Apache License v2.