As stated above, you can purchase Sissel from the North Peak safe house for 20,000....but you won't be able to access North Peak until you start Case 3......
An easy way to get this achievement done and over with (because I personally don't like this costume myself), is to go to the main menu, and "Case Select" Case Zero....it's a very quick trip to the very first investigation site. Keep pressing
to skip the cutscenes if you'd like.
Go through the level until you get to the part where you fall down the grate, after you kill the solo zombie, there is a POSTER right in front of you (if you didn't get it earlier.)
After you fight the zombies in the 2 cells, you are forced to change to your "X" costume, as soon as you change, and are forced to press
to heal, the door opens up, and you can run back to the Arcade and switch to the Sissel costume.
Just continue to kill all the remaining zombies, after all of them are dead, Vick will unlock the door, and your first investigation room will be right in front of you!! If you are worried about the "costume timer" running out, you can always run back to the Arcade, and re-select Sissel, and the timer will reset!!
Scan "Probably Not A Disco Ball", "Worst Dentist Trip Ever" (the "dentist chair"), then "Cure For The Common Plague", before the "costume timer" runs out, and the cheevo should pop!!
You can exit out now if you wish, but there is a Capcom Treasure very close by (if you missed it before, its kinda in a spot you would very easily over look)
Just exit out of the "investigation room", and go to the next (dark) room and watch the video, press
to skip if you'd like, go outside and IMMEDIATELY look left, the Treasure is up that little ramp......and that's it.....
Hope this helps someone!!!