how did itachi get the totsuka blade

Itachi’s Totsuka Blade is never given a clear in-canon origin. The simplest answer is that it appears as part of his Susanoo , and the series treats it like a legendary spirit weapon rather than something he physically found or forged.

What the lore says

The blade is associated with Itachi’s Susanoo and is usually described as an ethereal weapon with sealing power, not a normal sword. Black Zetsu also describes Itachi’s Susanoo, together with the Yata Mirror, as extraordinarily powerful.

What is not confirmed

There is no official canon explanation showing:

• where Itachi found the blade,
• who created it,
• or how he “obtained” it in a normal sense.

That is why fans often call it one of Naruto’s biggest lore mysteries.

Best practical read

If you want the most accurate short version: he didn’t get it like a regular weapon; it manifested as part of his Susanoo, and the story never fully explains its origin.

If you want, I can also give you the fan-theory explanations people commonly use for the Totsuka Blade and Yata Mirror.