Here's a link on EBay for some used ones.
http://cgi.ebay.com/ebaymotors/1979-1980-1981-1982-1983-750-CB750-ENGINE-GAURDS-/280583684494?pt=Motorcycles_Parts_Accessories&hash=item415417418eThey look a little banged up, but that's what they're for, right?
Better hurry: the EBay listing ends near midnight Monday.
I had a pair on my bike (check the picture on my profile), and I removed them a while back. I could be persuaded to sell them if someone wants them. They basically replace the existing engine mount bolts, and you'll need new bolts to make them work (if I can't find my originals).
Not even sure what I'd charge for them. Make me an offer + shipping. I'm in central OH.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.