Is there any way to get notifications about GitHub organisation events on some URL instead of checking notificactions with cron?
Not natively. The GitHub Notifications API does not provide a way to work with notifications in a "push" model, so the best you can do is pull them every so often with cron.
I am also not aware of any third-party services that would automate the task of pushing notifications by hitting a custom URL every time a new notification is pulled from GitHub, even though it would be moderately easy to make such a service.