How to Play Emulators on the Wii U (GBA, N64, PS1) – 2018
Homebrew Browser: http://wiibrew.org/wiki/Homebrew_Browser
List of Homebrew Emulators: http://wiibrew.org/wiki/List_of_homebrew_emulators
Here are a few sites you can visit to get some roms:
NOTE: Remember you need homebrew first! Once done you can also check out how to play Gamecube games on the Wii U using the homebrew.