Speaking as a notary in two states... most notaries should have a standard "official" looking affirmation page they can supply that can be appended as well.

The airline doesn't require it, except to the point at check-in if the country you are traveling to requires it, an agent who is doing their job checking Timatic will ask for it. Part of why I became a notary was to help out customers in such situations for my airline (and when it got around the airport that I was a notary, and the only one that wasn't one of the admin people with the airport authority meaning I was in the actual terminal, other airlines would call me). Of course I didn't charge passengers flying my employer...

But as wisely mentioned by others, it is better to have it and not need it to cover whatever eventualities may come up. A friend of mine on the Amtrak Cascades up to Vancouver last week said two adults with an infant got booted off the train prior to the border by the conductors because they didn't have a letter from the birth parent who wasn't there.