« on: February 28, 2013, 02:25:26 PM »
According to this site, there are 0.9 and even 1.2 gradual ND glass filters from Schneider. They seem to be 4mm thick and two of them can be mounted on a Lee holder.
I don't know how much this information can be trusted, but I figure it is worth mentioning.
Yes, Schneider Optics has an excellent selection of glass grad NDs. If you look at their MPTV line (motion picture television) they've got even more.
Just to clarify, though - the 'Lee holder' mentioned on the 2filter page is not the holder the OP has. Lee collaborated with Schneider to make a filter for their glass plates that holds two 4mm filters. The Lee Foundation Kit is different, designed to hold three 2mm filters.