Spin, just for reference... that appears to be a later model caliper. My '78 CB750K has the same caliper. The major difference between the early and late models is on the early calipers, the bolts run from the inside and thread into the outside half. On the later calipers (like yours) the bolts run from the outside and thread into the inside half.
I'm not sure when the design changed, but I know the '77-'78 models were like what you have. Sadly, to the best of my knowledge, none of the calipers used on the DOHC bikes will bolt right on.
Good luck with the hunt
It's been my experience that you can find rebuildable calipers for fair prices if you not in too much of a hurry. Most people seem to see them as college tuition for their kids though :
Oh, also... the calipers are the same from left to right, and the bracket and mount are interchangeable as well. OEM on the CB750 will have the caliper on the left, at 12 o'clock on the disc, in front of the fork. But it's not uncommon for people to flip them around and run behind the fork.
More... swapping the caliper mounts from on side to the other is not a
direct bolt on. It'll require a very little bit of work. This link is a very comprehensive article on how it's done...
http://www.dotheton.com/forum/index.php?topic=1525.msg11304#msg11304