This is currently the most popular and comprehensive library for 3DS content. They host games, updates, and DLC specifically formatted for easy installation. You can visit the hShop website and search for "Stretchmo" to find the DLC for the USA region.
You must ensure the DLC region matches your base game. If you have the North American version of Stretchmo, you need the USA DLC CIA; otherwise, the game won't recognize the extra levels. Stretchmo 3DS (USA) DLC CIA
A file is a package format used to install games, updates, or DLC onto a 3DS console using custom firmware (CFW). This is currently the most popular and comprehensive
Subreddits like r/3dspiracy have extensive megathreads and guides on how to safely find and install DLC for games that are no longer available for purchase. Safety and Requirements You must ensure the DLC region matches your base game
Many users have uploaded "Ghostlight" or eShop "Redump" collections to the Internet Archive. Searching for "3DS DLC CIA Collection" often yields results for the entire library.
Finding specific "CIA" files for Nintendo 3DS games like can be a bit tricky because the official Nintendo eShop for the 3DS has been closed. Since Stretchmo is a "free-to-start" game, the actual puzzles and levels were sold as Downloadable Content (DLC), which are now difficult to acquire legitimately. What is Stretchmo?
To use these files, your 3DS must be running custom firmware like Luma3DS. You typically install them using a tool called FBI . Where to Look (Community Resources)