Your best bet is to find a smashed Wii U gamepad someone smashed the screen on (has to be from the same country as the console because like the console, they region lock the gamepad!). It’s likely not going to be a common part and in addition to that, you don’t know if it came from a US or a Wii U gamepad from another country (Ex: EU) and if that’s part of the region locking scheme. Nintendo is incredibly secretive about how the Wii U gamepad is locked, while the console is a known software lock. If the model doesn’t match, it’s for a different region.
Check for a cheap one on eBay, get the module out of it and keep the rest of it for spare parts. The problem with Nintendo is it took them multiple generations of region free Sony/Microsoft* products to realize region locking died with HDMI when every TV spoke the same unlike composite where you had 60Hz (US)/50Hz (everywhere else) and a few other standards. Region locking made sense up to the Wii, but it would have died there if Nintendo wasn’t so stubborn.
*The 360 has a few locked games from Japan primarily, but 98-99% of the library is region free and the NTSC/PAL coding limits Xbox Live and DLC.