VintageCB750.com sells the fuse box for the 1969-1972 CB750K for $19.95. They have the fuse box for the 1973-1978 CB750K for $34.95. This same fuse box also fits the 1977-1978 CB750F Supersport. Also, they have the fuse box for the 1975-1976 CB750F Supersport for $44.95. This same fuse box also fits the 1976 CB750A Hondamatic. They even come with spare fuses!
On their home page, choose Electrical, then Wire Terminals Couplers, Fuse Boxes, Fuses. The fuse boxes are the first three items you see, just pick the appropriate year and bike model. I just thought I would share this information for anyone looking for one.
Also, when you install one, use some dielectric grease in the connectors for good connectivity and longevity.
It never hurts to replace the original fuse box with a new one as the fuse boxes tend to become brittle and corroded after all these years. At the least, a fuse box failure can ruin a long trip for you, and at the worst, I have seen bikes burn to the ground from severe electrical failure.