I generally find the main fabric and determine the position in the pattern that this fabric will take. Then I shop for the other positions in the pattern (from my stash). I have read the recommendation to make a black/white copy of the pattern first to take the color distraction out of the picture.
Like others who wrote, I have not bought the same fabric to copy a pattern. I usually do my own thing.
Value finders: You can also take photocopies of the fabric pieces which shows value.
A red vinyl binder divider works well for some values. Can't remember which though.