You can go to https://git.centos.org which is hosting all the sources used to build the system. Of course for RPM packages that is only metadata, not the actual upstream software source.
Just out of curiosity, since it sounds like you might know: on ubuntu I can do something like sudo apt-get source packagename and the system will just download the associated source code if a source repo is setup in the system's sources list. Does RHEL/CentOS have a similar capability?