I did the same thing on my last oil change on my 82 XS400 (11lbs of torque - lesson learned the hard way), and also have had no luck finding one on eBay, local wrecking yards, etc. There are a ton of the clamshell-type covers, but not these as far as I could find.
I was considering just brazing it myself (which I may do anyway just to give brazing a try), but that link from BBS360 seems to be exactly what I need. I think the brazing would work, but I will feel much more comfortable with a replacement.
I need to double check the part number in the manual when I get home, but it looks like the one.
Thanks BBS360 for the link - for whatever reason that online store never showed up in my Google search.