As nobody explained it yet, Nintendo closed their Data servers (NUS, Nintendo Update Servers), so we can't download IOS from the official network anymore.
you can get them officially from your own discs (in the update partition, with Wiixplorer for example), but wad could be old/outdated/not the version you want.
You can dump your own IOS from NAND to WAD (with BlueDumpMod)
You can get them from Modmii
or from not recommended places (piracy, download, etc.) but wads might be bad/wrong/fake/outdated pack of WADs etc. but you won't use the WAD to install them and replace the originals, you'll use the WAD as base for the patch and the result will be installed in desired slot, so no real risk.
Just don't mix Wii and vWii wad !
There's also the cIOS installer 2.2Mod by Fix94 which... get the source directly from NAND instead of getting them from WAD files ! No need to extract the title to WAD first.
Note that I (and probably nobody?) tried to use the 2.2Mod on Wii
(instead of v3.1)
It was useful on vWii because everybody had the correct version installed. You should be able to use it on Wii too (just don't use the vWii IOSes data), but if your IOS are outdated it'll use that version as base.
If anyone want to use 2.2mod on Wii, you need to replace the ciosmap.xml and the folders inside the installer with the correct version for your console.
I didn't know there were a new cIOS installer version with URL patched to private servers.