From: http://www.2filter.com
Here is direct link: http://www.2filter.com/4x4camerafilters/4x4ndgraduals.html
You will need to go to the section: 4x4, 4x5, 4x6 Graduated Neutral Density Camera Filters.
I have LEE Filters 4x6" Graduated Neutral Density (ND) 0.6 and 0.9 Resin Filter - Soft Edge