Where Ron and Hermione got a basilisk fang ??

They got it from the Chamber of Secrets, if wewe remember. After they kill the fang, Harry discovers that Ron tried and was successful at speaking Parseltongue, and that's how they got in The Chamber of Secrets.

After they got to Hogwarts (which, at the time was overrun with death eaters) Ron and Hermione both realized that they had a horcrux and had NOTHING to kill it with. So Ron suggested they get a basilisk fang from the Basalisk skeleton that was still in the chamber of secrets. Then they rush off to the chamber and then when they finally have one, they stab the horcrux (hufflepuff's cup), until it dies.