It's on the bottom of the 'About' tab on your organization's page. First, make sure you're managing your page through the business manager. For example, when we manage our Facebook page, we visit https://business.facebook.com/Rafflecopter/
Once you're managing your page, click on the 'About' tab below your cover photo.
Scroll down to the bottom of the page. Your Facebook Page ID will be the last item listed.