Wii U is cheaper than a mini PC capable of emulating it would be, unless you re-purpose something for your TV. Remember that streaming over LAN is an option!
As for where, anywhere you can find used tech is fine, but Japanese listings on eBay tend to be affordable.
Also remember you need Wii Remotes + a Sensor Bar for more than 1 player. Don't cheap out on Wiimotes, but feel free to cheap out on a Sensor Bar as it's literally just an array of IR LEDs.
Listings without GamePad are fine if you want to use Vanilla on e.g. a Steam Deck or modded Switch instead of a GamePad, but last I tested it, there was noticeable latency.
If you want an emulation station, then the wiiu is pretty good at that (supports every Nintendo console) and easy to hack
If you get a good price the wiiu is worth it. If you can find a wii at a good price, that's most the benefit of a wiiu right there.
I have a Wii, wiiu and switch and my wiiu has mostly been in a box for 10 years.