I believe you're talking about the chrome air cleaner covers, right? Those can be kind of hard to find in good shape. I may have some junk ones I can let go of. Keep an eye out on evilbay under Yamaha YG1. I think they're the same.
Edit: I just re-read your post. You want the carb cover for the side case, not the air cleaner. There was one on evilbay forever a while back, just keep an eye out. Seems like a lot them were missing for some reason.