From Surat Thani airport or town you can get a shared minivan to Khao Sok. Ask at the tour desk inside the airport terminal for this service. The journey takes around 2 hours and the cost is 350 thb per person.
From Phuket you can catch one of the public buses to Khao Sok. The public bus goes directly from Phuket bus terminal 2 to Surat Thani and stops in Khao Sok on the way. The journey takes 4-5 hours and costs about 200 thb per person.
From Krabi town or Ao Nang there are shared minivan services to Khao Sok, 350 thb from Krabi town and 450 from Ao Nang.
Private taxis are also available, please inquire for prices from your destination.