Finding games

Finding reliable sources for game files is more complicated than it sounds. This is because you may want to emulate games onto your DS that were never released for the DS, such as pokemon emerald being on the Game Boy Advance.

Each emulated game file will have a file extention matching the console the release is taken from, like the afforementioned pokemon emerald having the extention .GBA (Game Boy Advance). So, most sources will focus on or only be best for one file type.

Sources for CIA files

Brief overview of what a CIA file is

CIA files are, at their core, installable games in the format of one single file but can be from any console originally. Every CIA file is one game, so they're fairly easy to handle/store/install onto a console. If you would like to find out how to upload a CIA file to a DS, check this page.

1. Hshop is a go to and has direct safe downloads for CIA files, but won't have everything. This also comes as a homebrew app that will download to your modded DS during modding depending on the console, but the app is slow.
2. This bit of Internet Archive has quite a few CIA files for download and is a trusted site. I would recommend having a brief look here for a game if it isn't on Hshop before using the next source in this list.
3. Romspure is entirely safe so long as you are careful and understand how to navigate a shady website without clicking on popups. The only issue with this one is that it downloads the game as a 3DS file, but I still use this site to get CIAs, I just convert the 3DS file to a CIA with a method detailed on this page.

Sources for everything else

NDS files

NDS stands for Nintendo DS, and are subsequently Nintendo DS specific releases of games (think Mario Kart DS, Pokemon Black Version). They can be uploaded directly to a modded console without being converted to a CIA file.

Every other file type

Every other file type is best to be converted to a CIA file before putting it onto a DS/2DS (however DSi consoles are better off using something called GBArunner2 to run GBA files as they are. GBArunner2 will be covered in this page.)

1. Vimms Lair is the only website I really go to for a game that isn't CIA, be it NDS (which will be under Nintendo DS) or otherwise (under their respective sections in the provided link). It's reliable and safe and has good downloads, I would definitely recommend having a look around the site to see what it has to offer.

Small note

I just want to note that the legalities behind holding a ROM for a software release have been found to be completely okay, but I do not openly recommend having a ROM for a game you do not already own in some manner. Nintendo especially are very serious about their IP and I want to stress this for your safety.