I don't think direct bus is available from kolkata.usually people go via train.That is the best option or by car.But you can try a break route i.e kolkata to durgapur (varieties of bus services available in this route from volvo to regular buses).From Durgapur city center you would get a lot of buses for Bolpur.Infact you can get down at Panagarh (stoppage before durgapur)rather going to durgapur and wait for the Durgapur -Bolpur buses.but to get better seat suggestion is to go till Durgapur.From Bolpur, private vehicle or buses should be available for shantiniketan.In this route you can also get down at illambaza and from there also bus or private vehicle should be available for Shantiniketan., but my suggestion if not going there by car try train.