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B+W, Heliopan and Tiffen have 67mm grad ND filters. Have a look.
That said I really recommend against round screw-in GND filters. The graduation is always dead center at the "equator," and that's rarely where you want it for optimum image composition. A rectangular filter lets you adjust where the graduation falls in either vertical or horizontal compositions for optimum placement. You can buy a Tiffen GND in Cokin P size plus a Cokin filter holder and Cokin 67mm P-adapter ring and have a much more versatile rig.