Easy way: get a usenet subscription and go from there.
Glib remark over:
Alas most file recovery tools are not designed for filesystems outside their little group (FAT, NTFS and the common linux ones are usually covered by freeware) as you say.
Good news is WBFS is fairly simple and and due to the way things work fragmentation is likely to be minimal/non existant and wii isos are fairly predicatable things and better still they are signed (even fakesigning, if present, should be enough to perform error detection courtesy of the cascade effect).
It would be better if the person doing it knew the WBFS file system through and through, alas I do not and I do not feel like poring through source code right now.
Before you start if you can do it get a sector by sector image of the drive or partition do so and try and work with that, it is likely a lot of space though so you will probably have to end up doing it "in the real world". If nothing else dump as much as you can from the disc to give you something to work with.
I am not sure if there is an "protocol overhead" (a header attached to each sector or something) with WBFS but I do not imagine there to be anything special.
Find yourself 3 things:
A hex editor capable of reading and writing raw drive sectors, most good ones can do this. If you want to go freeware then:
http://mh-nexus.de/en/hxd/
A tool capable of reading raw drive sectors to files on your new disk. Afraid I have none to hand right now, sure you can use a hex editor but it will probably take ages.
Some way of assembling files. This is simple enough, in windows use "copy /b" (as in "copy /b file1.ext+file2.ext+file3.ext filecomplete.ext") and unix based systems can use "cat".
Best case scenario:
something in the boot sector/initial file table/similar got messed up. You correct it/bodge something (format another drive as WBFS and get an idea if you do not fancy/are unable to pull apart source code).
Getting worse:
You have irreparable damage to a few things, you can rescue some data but not all of it.
Here you will have to piece together isos.
Even worse:
Extensive random corruption. Not much you can really do that will be worth your time.
In all three cases you ideally want to be able to lay your hands on the file allocation table, this is basically a long list of what file each sector contains. If not as I mentioned Wii isos are predictable and as such can be probably be pieced back together if they even need that.
