Hi nana
I prefer Sombattour .In the Monday morning (9/2/2009) it still have ticket. You can buy it at Morchit and it has Departure Times 7.00 am: Bus Fare 610 ฿ for 1'st Class and Departure Times 6.30 am: Bus Fare 475 ฿ for 2'nd Class
Don't worry Monday morning the seat isnt full and you can see example bus at this link
http://www.rottourthai.com/showthread.php?t=46