Amazon.com : NiSi 100x100mm Neutral Density Extreme Filter Kit Includes ND 0.9 (3-Stop) Filter, ND 1.8 (6-Stop) Filter, ND 3.0 (10-Stop), and ND 4.5 (15-Stop) Filter : Electronics
Photography - A guide to N.D. filters
What is an ND Filter: A Detailed Guide to ND Filters in Photography
Amazon.com : Polaroid Optics 72mm Neutral Density Filter [ND 0.9] Compatible w/ All Popular Camera Lens Models : Camera Lens Neutral Density Filters : Electronics