Nowhere. The series is licensed. Your only free legal options are ad-supported streaming (like Pluto TV or Tubi), which do not allow downloading—only streaming.
If you buy from iTunes, you get .M4V files (with DRM). If you rip Blu-rays, you get .MKV files (no DRM). Streaming apps use encrypted cache files (cannot be moved). Download All Naruto Shippuden Episodes English Dubbed
If your goal is to watch on a plane or without an internet connection, here's the safest method: Nowhere
Finding official ways to "download" usually involves using the offline viewing features of paid streaming apps or purchasing digital copies. If you buy from iTunes, you get
For many Western fans, the English dub on Cartoon Network’s Toonami block was their introduction to the ninja world. Downloading the dubbed episodes is often about recapturing that specific nostalgic feeling from childhood viewing.