The direct bus from Kandy to Sigiriya leaves from the Central Bus Station in Kandy, Map Location: 7.290085, 80.631245 , at 07:35 and costs 160 LKR, less than 1 USD. It takes about 2 hours to get to Dambulla and then another 45 minutes to get to Sigiriya. The final stop is at Map Location: 7.951692, 80.755905 .
There are also buses that leave every hour from Kandy to Dambulla and then every 30 minutes from Dambulla to Sigiriya.