Well it's certain that he had Hashirama and Tobirama's DNA pre timeskip.
His earliest known use for Hashi's DNA, Yamato, was more than a quarter of a century ago.
He might have gotten Hiruzen's DNA during their battle, again, pre timeskip. If that's not it, Hiruzen was his sensei, he must have at least spent more than a decade or two with him. He could have stacked up on his DNA even back then. :shrug:
I don't know about Minato, but Oro being Oro, he could have easily obtained it somehow.
There was probably blood splatter on the site of his final fight with Kurama and if that's not it, Kabuto explained how he obtained so many DNA samples; graverobbing.
If other villages didn't dispose of the bodies of their boss fighters and Kage's, I doubt Konoha did too. :shrug: