Legally? I've been tracking down physical cartridges and using a ripping tool, which is basically a hardware slot for a gameboy cartridge that connects to a PC via USB. The one I own personally is this one, which can dump all the common cartridge types ("mappers") and their save data:

https://bennvenn.myshopify.com/products/reader-writer-gen2

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:

https://pdroms.de/news/gameboy

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:

http://tasvideos.org/EmulatorResources/GBAccuracyTests.html

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!

archive.org has an archive of various retro console roms in their "No-Intro Collection 2016-01-03 Fixed" collection.

Being copyrighted, it's not legal to redistribute them. You're favorite torrent search engine might be able to help, though. Or you can buy them off eBay and dump the image yourself. I'm not sure what the current state of dumping hardware is, but 10 years ago I hacked together one that used a PC parallel port.

I can highly recommend https://www.tindie.com/products/insidegadgets/gbxcart-rw-gam....

> Works with Windows XP or higher and Linux/Mac (console version). On Linux/Mac the speeds are much slower.

How come?

> You'll just need a Micro-USB or USB-C cable depending on the connector type you choose. Please note the USB-C connector only has the USB 2.0 lines attached so it may not work with devices expecting a USB-C capable device; you might need a USB-C to USB-A adapter and then a USB-A to USB-C cable.

Do you know if there are plans for a future revision of the hardware which will allow for the greater speed that USB-C can provide to be utilized?

>> Works with Windows XP or higher and Linux/Mac (console version). On Linux/Mac the speeds are much slower.

Not sure. It dumps the cart over serial and there were some problems with higher speeds at one point.

> Do you know if there are plans for a future revision of the hardware which will allow for the greater speed that USB-C can provide to be utilized?

I don't think the carts themselves are that fast, there wouldn't be a huge amount of benefit. At least when I tried it it wasn't even coming close to using full USB 2 speed.