EBay has some right side covers for the 79-82 models for between $40 and $100. There's a SS right side cover for $70. You might try some of the MC junkyards out there. There is a place called Kentucky Cycle Salvage
Here is their website. You'll need to register to get into searches and auctions, but it's free and worth it.
Good luck, and I hope this helps.
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.