Have you tried eBay or OfferUp? You might even find some at your local Goodwill in the electronics section.
That said, even if you're having trouble finding unmodded Wiis, there are guides that cover unmodding modded Wiis, like this one:
I am curious why one would want to do this though: Modded Wiis provide more features, and the only reason one wouldn't want to mod them is to prevent getting banned, unable to connect online.
However, the Wii's online features are (ironically) now only available by modding the Wii, because Nintendo pulled the plug on their Wii online services: modders provided alternative networks to connect to.