The notification-controller is part of the flux gitops toolkit components (docs). It is recommended to use this component in conjunction with the remaining toolkit components during install.
Although there are multiple methods of installing, the example below demonstrates using the flux cli, also available as a chainguard image (cgr.dev/chainguard/flux):
NOTE: Installation assumes a properly connected kubectl context.
# Using a pre-installed flux cli
flux install --registry cgr.dev/chainguard
# OR using the provided flux chainguard image
docker run cgr.dev/chainguard/flux export --registry cgr.dev/chainguard | kubectl apply -f -