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?
You cannot install CIA files on a "stock" (unmodified) 3DS. If you haven't modded your system yet, the gold standard guide is 3DS.hacks.guide . Stretchmo 3DS (USA) DLC CIA
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
A file is a package format used to install games, updates, or DLC onto a 3DS console using custom firmware (CFW). You cannot install CIA files on a "stock" (unmodified) 3DS
Because the eShop is closed, new players can no longer buy the DLC officially. This has led many to look for archived CIA files.
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
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)