Hi... harsh you can go to spiti through two ways 1 from manali you can go to spiti through by bus 2 through Shimla you can catch a bus to spiti so decide which route is fine for you
There are buses from Shimla, Manali and Kullu too. There are two routes. Delhi Shimla Kaza route Delhi Manali Kaza route. Since journey time is too long you could break the journey accordingly. Volvo buses, HPTDC and private buses operate to Spiti valley.