The Psijic Motif chapters (or very rarely, the full book) will come from adventuring around Summerset.
Activities that may reward a Psijic Motif chapter are, in order of most to least likely:
Completing Abyssal Geysers
Killing World Bosses
Completing Abyssal Pools
Treasure Chests (with higher-difficulty locks being more likely, and easier locks being less likely)
Killing Delve bosses
Killing monsters
The chance to obtain a Psijic Motif chapter or book from these sources increases with your rank in the Psijic Order skill line, with Rank 10 giving you a ten times increase over those not versed in the ways of the Psijic Order.
The Style Item for the Psijic Motif, Vitrified Malondo, drops from the same sources as above, more commonly than do the Chapters.
With rank 10 in Psijic Order I've got from all the time since Summerset launched, only 2 Psijic pages, and both were from the Psijic portal. They must be incredibly rare.
Life is a ride, like days in a train, cities rush by, like ghosts in the night. The rhythm of wheels, time fades away, stations of a journey, destination unknown.
There are none for sale in the guild stores. I have psijic 10 on every character except my handbag and I didn't even know these motif pages existed until I got a sealed writ demanding one. Now THAT seems a little unfair, but I suppose they have sealed writs for gold jewellery so that is even worse.
Favourite NPC: Wine-For-All
Mostly PC-EU , with a lonely little guy on NA.