Based on this answer.
I do know that old Wii demos are removed from Wii shop, is there any place I can download them?
P.S. I have "original" Wii, so pirated versions won't work here and I'm not interested in pirated versions of the demos.
Wii demos can only be downloaded from the Wii Shop channel. If a demo is no longer in the Wii shop, there's no way you can download it.