The Caffeine Informer website has a great deal of helpful information. In particular, the comments at bottom of the "Caffeine Withdrawal Symptoms: Top 15" page are very helpful!
The Reddit /decaf community has a great amount of content on caffeine withdrawal and quitting caffeine. Despite the name, it is more about being caffeine free than drinking decaf (which still contains some caffeine)
Unfortunately the Caffeine Symptoms page is no longer live, but the text of one person's story can be found in archive form here. We also have a PDF copy which we can share if this link does not work
The Quitting Caffeine Facebook Group has posts about quitting caffeine
Caffeine Withdrawal - The Truth is another Facebook page started by someone who struggled with caffeine withdrawal
A short article about the science behind the headaches
Benefits of being caffeine-free