Exactly this. Use wiiu.hacks.guide.
There is a method to use Mocha CFW instead of Haxchi if you insist of not buying a DS game. Other methods might show up in the future.
Read the guide carefully and: Haxchi requires buying a DS game. No way around this!
Whatever you do, under no circumstances try to use Haxchi on a DS game, you didn't buy. If you use the cold boot variant of Haxchi (CBHC) you will permanently brick your Wii U if the DS game isn't legitimately bought from eShop.
Standard Haxchi will simply not work this way. CBHC will fully brick the Wii U without legit DS game(Yes, I have to use red colour here. I don't want more Wii U consoles bricked)
The main purpose of custom firmware on Wii U is: Signature Patches → This allows starting software that does not have a valid signature. Haxchi does this once loaded. But before loading Haxchi the Wii U will only start properly Nintendo signed software. See: With an illegitimate DS game you're running in circles. Haxchi would need to be already loaded to start the Haxchi title.
Why is this such a problem? This question comes up regularly. I admit the problem is not apparent for a beginner. One has to think a little and needs to have a general idea what software signing is. But: All the guides clearly state that "pirated" DS games will not work. This fact is not hard to understand even if one doesn't understand the reason behind it.