Singularity Container Registry at https://singularity-hub.org
There are also some good articles on other sites with help on using singularity which are listed below. Note that some of these cover the older singularity version 2.2 or are written for Debian based distributions. Read the current Singularity User Guide.
High-Performance Computing at the NIH: https://hpc.nih.gov/apps/singularity.html
Case Western Reserve University:
See the section “Installing Singularity & Managing Image in Your PC (Linux)”
A tutorial at Github, “Creating and running software containers with Singularity”:
PDF presentation at the HPC Developer Conference: