EDIT: Alright, I found the one I was looking for and this is what I had to do to figure it out. I went into the game and went through the list of names of the posters I already had. I used this website:
and just wrote down every number, the number the website linked used, to find out which person I was missing and looked at their location description.
Since the description isn't very specific on that site, I used:
to see what posters where in the areas I needed, and watched the videos on them. Long obnoxious process, but it worked.