The house motifs drop from pretty much any NPC in my experience. In 60-90 mins of pick-pick-stab I usually pick up 2 of the house motifs.

The Buoyant Armiger motif chapters are an extremely rare drop from treasure chests. I've gotten about 10 so far, but I've also gone weeks without seeing any.

Speaking of the three house motifs only, the strategy that worked for me is to max out Thieves Guild and Legerdemain and take all the passives. I'm not sure that improves your drops but it seems to and with all the stealing it's a worthwhile investment anyway. You've got to pickpocket them until empty (sometimes you get it on the second one, but usually it's the third). Usually I get a motif within the first six NPCs I try, though sometimes it takes longer.

Once you've done that, stop. There seems to be a one-hour cooldown that either prevents further drops, or makes them very unlikely. Do something else, then after a little over an hour, come back and begin again. You should see another motif drop relatively quickly.

During that hour, you're free to switch to another character and have them farm motifs. I have three characters with maxed Legerdemain and Thieves Guild and swapping between them showed the cooldown was character-specific.

One other weird thing, though, is that my odds will sometimes plummet through the floor if my account has gotten too many. I have no explanation, but noticed that after a while all the characters start failing to get motifs. At that point I just quit for about five hours or so and stick with just one character.

This method allowed me to get all the motifs for the three houses within four days. Hope it helps!