The bus station is about 2km out of Kamphaeng Phet. To get there, catch a songtheaw from around the intersection of Kamphaeng Phet and Tesa Roads, which takes you over the Kamphaeng Phet Bridge and 500m further to the bus station.
Buses come and go all day long from the main bus station as Kamphaeng Phet lies on the major Bangkok to Chiang Mai route.
Bangkok (Regular) leave at 7.30, 8.30, 9.30, 13.00, 14.30, 15.30, 16.30 and 18.30 and take six hours.
Bangkok (VIP) leave at 10.00, 12.00, 13.30 and 19.30, take five hours.
Chiang Mai (VIP) leave at 11.30, 14.00, 15.00, 15.30, 19.30 and 20.30, and take six hours.
Tak (and then onwards to Chiang Mai) leave at 6.30, 8.00, 9.00, 10.00, 11.00 and 13.00, and take one hour.
I took a bus from here to Lampang which took 3.5hrs, after stopping in Tak along the way. It cost 180 baht.
There's goverment bus which run from BKK to Kampaeng Phet but just 2-3 times a day. Other private bus companies offer more trips but always a half trip from Chiang Mai,so it sometimes fulls. The recommend private bus company is Sombat Tour which offer a bus with sitting seat on the 2nd floor and mini sleeping private room on the 1st floor. The fare is 270 THB with 1 meal and beverages along the trip.
Kampaeng Phet Bus Terminal located on the West outside the city. It takes about 3-5 minutes from city center. At the terminal,you can find some counters of both goverment bus and private bus company. From this terminal,there 're some songtaews or 2 row mini truck which run to some main towns in Kampaeng Phet. Also,there're some busses run to Sukhothai,Phitsanuloke and Chiang Mai.