Hello CyrilC - just bought an '82 CB750c myself which has a pretty awful right-side cover. It seems that replacements of this side cover, in particular, are hard to come by. I bought one off eBay the other day from a guy in the UK who has made some himself and has more to sell (~$95 before shipping see
http://www.ebay.ca/itm/120919433593?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649). If you're interested I'll let you know what I think once it arrives...
Update July 19
The replacement cover arrived and it's quite nice. It will need painting (it's made of that stark-gloss white boat-hull gelcoat material) but it is quite robustly made (it's heavier than the original). This definitely appears to be a reasonable (and available) alternate to finding an original replacement in good shape. Only downside is that it doesn't come with the emblems (although the holes for the emblems are pre-drilled)...
1982 CB750C