I have been using ND graduated filters for a while now. I never used a holder, I always would hand hold them. It's not very difficult to do but I did find situations that I would of loved having a holder. Long exposures for example, or situations where it required precise placement. It's also a bit cumbersome when I compose the scene with a cable release and a filter to hold onto. So I started to research filter holders. It came down to a lee or a cokin. I chose cokin because of this review posted by Darwin Wiggett.
Singh Ray makes a sprocket poloarizer that will fit the Cokin Z Pro Filter holder. This way I can have one polarizer that I can use for all my lenses. All I have to do is get an adapter ring for each lens. You can pick up the filter holder at any camera store but I will have to admit they are hard to come by. Once they are in stock they sell out very quickly. This is due to some supply issues.
Check out B&H and click on the "notify when in stock". When it's back in stock, B&H will email you, but act fast because it's first come first serve. It took several weeks for me to finally get one, but I am completely happy with it.