Nothing takes me back to my childhood like the classic ninja anime—those shows were my gateway into the world of animation! If you're hunting for gems like 'Naruto' or the older 'Ninja Scroll', Crunchyroll's got a solid library, especially for subbed versions. Funimation's another great spot, especially if you prefer dubs; their catalog includes classics like 'Basilisk'. Hulu's surprisingly decent too, with a mix of newer and older titles, though you might need a premium subscription for some.
For more obscure stuff, I’ve stumbled on treasures on RetroCrush—it’s free and specializes in vintage anime, including lesser-known ninja series from the '80s and '90s. Tubi also has a hidden trove of classics, ad-supported but totally legal. And if you’re willing to dig, YouTube sometimes has official uploads of older episodes, though quality varies. Just be wary of shady sites; nothing ruins nostalgia like malware.