Finding a title key for Wii U USB Helper can be a bit tricky, but there are a few methods to obtain one:

When the application asks for a "Title Key site" or "Ticket site," you can try the following widely-used addresses: titlekeys.ovh wiiu.titlekeys.gq (often requires a hosts file modification to work)

As you may already know, Wii U USB Helper has been discontinued, however, this shouldn't mean we're not allowed to use it anymore.

If you are looking to play Wii U games today (for legitimate backups of games you own), the community has moved on:

A: If you lose your title key, you may need to re-extract it from your Wii U console or obtain a new one from a reliable source.