You can find IP addresses of AlertSite monitoring locations on this page:
https://www.alertsite.com/cgi-bin/helpme.cgi?page=monitoring_locations.html
AlertSite updates this page daily based on the information it has in its database. So, this page contains the most actual information.
Note that the firewalls running on the side of your checked website, email server or other resource should allow connections from these IP addresses.