Buses run from 4.00am every 20 minutes daily with the last bus leaving Matara for Colombo at 9.40pm.
Tickets can be bought from the conductor on the bus, so no need to purchase in advance. One way Rs.550.
You can call 1919 for more information.
Makumbara bus station is to the South East of Colombo City Centre, to get there from the centre of town you can take the 112 and the 138 bus