To be fair, this is Game Boy modding we're talking about. It's all 'for the heck of it.' I personally like this stuff more than seeing people buy DMG's in bulk, slap in a bunch of off-the-shelf mods, sell them on ebay, and call themselves modders. I think I get it though. I wouldn't actually want to play the DS-in-a-DMG, more than a regular DS.
![Why Why](http://www.retrogamenetwork.com/wp-content/uploads/2019/01/wireless-gb-controller-gbp.png)
The popular titles (especially Pokemon) can be hard to track down at a fair price, but lesser known gameboy games can be found easily on Amazon and from other used sellers for a few dollars each. Even stuff like Tetris is easy to get ahold of because so many copies were produced. Bad games still make great test cases. :D
If you're after homebrew there's a whole bunch of sites that host it. One site that I pulled my early test ROMs from was:
But there are of course countless others. The demo scene is quite lively, and once your emulator is working at a basic level, demos can be good test cases since they often use the hardware in surprising and unusual ways. Finally, once you're serious about accuracy, check out this list of test ROMs:
Especially the tests by blargg; these are designed with a simple pass/fail result but go seriously in depth checking out hardware edge cases, and the source code is available which makes debugging them a breeze. Finally passing his CPU test was a major milestone.
The rabbit hole is very deep, but it's a fun dive!