https://github.com/davFaithid/CIA-to-3DS-Rom-Converter
https://github.com/3DSGuy/Project_CTR/releases
This converter requires a version of makerom, which can be found here.
This converter only works on Windows at the moment.
Upon downloading makerom, create a dedicated folder for makerom.exe and Converter.bat, and move makerom.exe and Converter.bat to that folder.
Drag your CIA to Converter.bat and you are good!