Sage Mode can be learned in two places, Mount Myōboku of the toads and Ryūchi Cave of the snakes. Learning Sage Mode at Mount Myōboku or Ryūchi Cave grants the user with a toad and snake style of Sage
Source: Sage Mode - Narutopedia
After Orochimaru's sealing, Kabuto travelled across the world and ultimately discovered the location of the Ryūchi Cave. There, he was taught senjutsu by the White Snake Sage and gained access to Sage Mode. Physically, Kabuto undergoes little alteration in this form, with the only visible changes being the markings around his eyes extending onto his back and the growth of four horns on the back of his head, as well as darkened sclerae.
[...] Kabuto also gains access to further snake anatomy, such as their brille, which allows him to severely impair his opponents through the use of intense light whilst remaining completely unaffected himself. [...]
Due to his research and self-modifications based on Jūgo's clan's abilities, Kabuto's body can passively and continuously absorb natural energy without aid even when moving, allowing him to potentially maintain his Sage Mode indefinitely. According to Kabuto, this meant that he was no longer a snake, but had instead transcended into a dragon
Source: Kabuto Yakushi - Narutopedia (emphasis added)