The filenames for the Dutch subs are: SW-nld-full.sup, ESB-nld-full.sup, and ROTJ-nld-full.sup. They can be found in the SUP-720p or SUP-1080p folders.
EDIT: Many preservations including the Despecialized Editions only include the SRT portion of Project Threepio. If you want BD-SUP, you can download the whole project, typically from the same places you got Despecialized.