Where can I find an online list of CentOS packages and their versions?
I started with this, which got me a list of CentOS mirrors, and then picked the first mirror.
Here's a list of packages for CentOS 6.
If the particular mirror lsited here is dead, you can simply find another mirror by returning to the list of mirrors and follow the same logic.
This mirror seems to be kept up-to-date: http://mirror.centos.org/centos/7/os/x86_64/Packages/
See https://pkgs.org/ which allows you to search a range of CentOS (and other) distributions.