Here are the nightlies:
https://github.com/vyos/vyos-rolling-nightly-builds/releases
Here is the latest nightly:
https://github.com/vyos/vyos-rolling-nightly-builds/releases/latest
And here is a script you can use locally on your VyOS (or elsewhere if you just want to download the latest iso):
#!/bin/vbash
# Include VyOS functions
source /opt/vyatta/etc/functions/script-template
# Script debugging
#set -x
# Set variables
VRF=INTERNET
#read -p "Enter path to update-file: " URL
#URL=https://s3-us.vyos.io/rolling/current/vyos-rolling-latest.iso
URL=$(ip vrf exec ${VRF} curl -s "https://api.github.com/repos/vyos/vyos-rolling-nightly-builds/releases/latest" | grep browser_download_url | head -n 1 | cut -d\" -f4)
# Perform stuff
run add system image ${URL} vrf ${VRF}