Here are the locations. Just like on the Elyos side, there is a shugo for the top and bottom, and another shugo for the boots/gloves and pauldrons.
If u wanna see how the armor actually looks like check here: Asmodians - Elyos Skins
on my blog.
Thats only the level 30 skins.
If u want the level 50 skins, I bet they are in the Instance in Brustonin, so should not be hard to find.
Now I don't know the timer of the NPC. But perhaps they are on the same times as on the Elyos side. If so, then these are the times:
2nd, 19th and 28th of the month from 1am - 4am
6th, 10th, 14th and 23rd of the month from 12pm - 4pm
And that should only be for the shugo selling top and bottom armor pieces. The one for boots/gloves/pauldrons should appear every day, at dusk and at dawn, and sometimes in the evening.