Use GameObject.FindGameObjectsWithTag and then Random.Range using '0' and the length of the GameObject array.

Read through the documentation for both, and if you're still having trouble, let me know and I'll update my answer with further details.

Updated...

if(freeRoam == true)
{
    var allStones = GameObject.FindGameObjectsWithTag("Stone");
    randomTarget = allStones[Random.Range(0,allStones.length)];
    disFromRTarget = Vector3.Distance(this.transform.position, randomTarget.transform.position);
    transform.LookAt(randomTarget.transform);
    speed = walk;
    transform.position = Vector3.MoveTowards(transform.position, randomTarget.transform.position,  Speed * Time.deltaTime);
    animation.Play("Walk");
}

Actually just calling you variable "RandomTarget" doesn't make your target random at all! :D

When you call FindWithTag function you get just the first element with that tag (I fought with that function just this morning! :D)

You should first get the whole array of GameObjects with the wanted tag, like this...

var myStones = GameObject.FindGameObjectsWithTag("Stone");

And then for every object which must point to one of those you can choose a random target...

var targetStone;
targetStone = Random.Range(0, myStones.length);

That should do the trick! :)