For getting from Phnom Penh to Pailin please have a look on the Battambang Province Page. Coming from Battambang is the only wise thing to do, except if you're coming from Thailand over the international border crossing. Pailin itself lies about 83km southwest of Battambang and is just 19km from the border with Thailand. The Major Road 57 from Battambang to Pailin has gone to pieces and makes it a pretty hard ride. The road can only be managed by smaller lighter cars such as minibuses, pick ups or share taxis due to its wimpy condition, but its still better than it was a couple of years ago, with many new bridges. The scenery along the road heading to Pailin is nice and there are a couple of interesting places on the way. One is Phnom Sampeu, a mountaintop temple, which has memorials set up with skulls and bones of Khmer Rouge victims. It's located 15 km from Battambang on the Pailin road. Security is not a problem. Never go off the street as there are many mines still remaining in the ground. The ride from Battambang to Pailin (4hours) with a share taxi or minibus costs around 160Baht and with a pick up 120Baht (inside) or 80Baht (on the back).
You can book a Private Taxi online in advance for a few dollars more.
There is also a breathtaking tour from Pailin to Krong Koh Kong (South) for hardcore bikers with plenty of off-road experience. It starts in the Treng district, just 25km East on the road to Battambang. Than it runs down South through former Khmer Rouge strongholds such as Samlot and Veal Veng, which is 275km from Pailin and the last place to refuel. Good luck!
Bus Ticket Online is available and you can check below: