Looks like any written source: (http://www.sageadvice.eu/2016/04/26/book-of-shadows/via Twitter):
Matt Harrah: "...For Book of Shadows warlock: what constitutes "finding" a ritual spell? A book/scroll? Observation of cast?"
Jeremy Crawford: "...Finding one written down."
A wizard's spell book, another warlocks Book of Shadows, or a scroll all qualify. I could see a DM making one carved on a wall of dungeon, etc, and it would count RAW and RAI.