You can build .NET 9.0 from the repository by cloning the release tag v9.0.114 and following the build instructions in the main README.md.
Alternatively, you can build from the sources attached to this release directly.
More information on this process can be found in the dotnet/dotnet repository.
Attached is the PGP signature for the GitHub generated tarball. You can find the public key at https://dot.net/release-key-2023