For the actual clothing of it I used everything from my regular Hakkai version so that costume will have all of that info. Here I'm gonna elaborate on the fun shiny things that make the Youkai version epic.
The ears I bought from an online store and am very happy with them. If you're curious as to where I got them just send me a PM...since I'm to lazy to look up the link now. I blend them into my actual ears with liquid latex and then hide it with my foundation so you can't see the seam.
The vines I hand paint on myself and are all acurate to the manga...even the ones on my hands (although I need to extrapolate for the palms a bit). It takes a couple hours but I'm really happy with the results.
The wig is a Fashion 101 in their dark brown from amphigory. It's the exact same one as what I used for Tsuzuki...and had to buy a new one when I had used it for him. 9.9 I'm really happy with it. I trimmed and styled the whole thing myself...and even manage to get that flip in the back where the shorter hair on top stops and the long mullet-y part begins.
I bought some yellow cat-eye contacts and only wear the one since only his left eye changes...a little something extra. Although it constantly rotates on me and I have to fix it. >.<
Got some long fake nails too...that way I can has sharp claws. The only drawback being that you literally can't pick up anything while wearing them.
And last but not least is a little detail that I included that I feel makes my youkai Hakkai unique...I HAS VINES! Because he's able to make the vine markings spread across what he touches, I went to Michaels and got some fake vines and green wire and wrapped the wire around the vines and sewed it onto an elastic loop. That way when i wear it under my sleeve it looks like the vines come out from me...and because of the wire I can pose them and wrap them around things! XD