We made a video about the basics of ND gels the other day, and given that we don't have a grip truck to haul stuff off of we had to go actually buy gels from the photo supply store. I took the opportunity while there to grab another of my favourite things on the earth, a little sample-book put out by Lee Filters (Rosco makes them, too) containing little swatches of all the gels and scrims and reflectors that they make, which is a lot of different gels, scrims and reflectors.