There are air-con busses leaving from Mo Chit Northern Bus Terminal in Bangkok every 30 minutes, daily from 7 am to 11pm. To get to the bus terminal, take the MRT underground to Kamphaeng Phet Station and from there a motorbike taxi (30 baht) to Mo Chit Bus Terminal.
Once you arrive at the Sukhothai Bus Terminal (see picture below) there are songthaews (shared pick-up taxis) taking you to the foreign hotel area in New Sukhothai which is located about 2,5 km drive from the bus station. One way fare: 50 baht. They also drop you off right at the door of your hotel if you ask them to with no additional charge.