Grade 6????????? Who sells a grade 6?
Any hardware store or auto parts store. One of the easiest grades to find. It will be harder to find grade 8. As I said I was just guessing what bolt was holding the blades on and I get most of my torque specs from a chart which is by grade and size.
There are no grade six fasteners, at least for any type of normal use. Grade 2 or 5 are easiest to find, grade 8 not that difficult.