Ziperto is widely regarded as an excellent destination for decrypted 3DS games specifically intended for Citra. The site organizes its library clearly by console, making it easy to browse and locate titles. Its direct-download links are straightforward, though as with any ROM site, using a good adblocker is strongly advised.
Avoid generic ROM download sites that pop up on the first page of search engine results. They often employ dangerous tactics: where to find decrypted 3ds roms
A simple "drag and drop" tool for Windows that batch-decrypts files for Citra. Ziperto is widely regarded as an excellent destination