Both motifs are a part of the Dark Brotherhood DLC.
Dark Brotherhood motif chapters are rewarded for completing Sacraments for the Dark Brotherhood. More info here : en.uesp.net/wiki/Online:Dark_Brotherhood_Style
Assassin's League motif chapters drop from world bosses and delve bosses in the Gold Coast. More info here : en.uesp.net/wiki/Online:Assassins_League_Style
If you don't have the DLC you can buy from guild traders or crown store. As far as I remember, the Assassin's League motif chapters are fairly cheap at guild traders. Around 600 gold per chapter on PC/EU. The price has gone up a bit but still they are quite cheap compared to other motifs.
Yes I do have the DLC, and those Motifs look really cool, so I guess I'll just have to wait a bit to unlock the Dark Brotherhood one, and find a group to get the Assassin's League one !
Ok I'm "uping" my own thread, @TheWhiteDarkness (or anyone else) do you know where we can learn the "Soul Shriven" style ?
I just got a shield from this crafting style and it's absolutely awesome !
I didn't see it on the Crown Store though, and have no idea where to find the motif book
"The Soul Shriven Style is a reward for completing Cadwell’s Silver Quests. These quests are given after you finish the story from your Alliance and are VR1 to VR6. You will learn the entire Motif after you finish Cadwell’s Silver."
-> http://esoacademy.com/soul-shriven-style/