Neutral-density (Solid)
From Amazon
Lee Filters Big Stopper Neutral Density Filter Disc for SW150 System - 1000x / ND 3.0 / +10 Apertures
Clifton Cameras
LEE Filters SW150 150x150mm Big Stopper Neutral Density 3.0 Filter
WEX Photo Video
Lee SW150 Mark II Big Stopper