Well, let's see here... I'm making my own Shion outfit right now, and learning a lot. >_o
I don't know if it's entirely possible to find the pattern I used for the mini-jacket, since it's probably pretty old -- McCall's cut-to-fit 5136, View A. Might want to scope out thrift shops to see if their pattern section has anything vaguely resembling it.
I made a muslin, and then altered the basic shape (scalloped the sleeves, cut in the 'lapels', brought up the hem about two inches and cut the curve in the back). From there, I used my dress form (which you may or may not have, eep) to wear it, and started sketching out where I figured all the pieces were, with the help of a measuring gauge and *lots* of printouts of Shion. >_o Your methods may vary.
Once I had the stuff sketched, I took blank sheets of patterning paper (I was ghetto and just taped scraps together, eheh ^^
, draped 'em over the form, and traced the designs with pens or washable markers - then I added margins of allowance to the pieces (usually 5/8ths will do) and cut 'em out.
I nabbed an 'upper' collar pattern from a different set (I don't remember the number, but nearly any jacket should do, I hope.)
I hope this helps in some small way. Or that it at least made sense. ^^;