Answering my own question: AFAIK there is no 'proper' EL9 repo hosting libc++ packages
There is, however, a way to build the RPMs so they can be self hosted. I believe this [1] github repo has basically taken the RPM sources from upstream (Fedora) and made them available to build for EL9. There are also binary packages for x84_64 in the github release section, but it's probably not wise to trust those, and just build the RPMs yourself.
I'd be happy to retract this answer if there was a 'proper' EL9 repo to avoid the self build and host option. I'd also be interested if anyone knows the reason for the fact there is no official EL9 libcxx package.