You can get these long distance busses from here ,
42 Colombo - Deniyaya / Beliaththa Kumbura / Morawaka / Opatha / Udugama / Neluwa / Thiniyawala / Mapalagama
(via Dehiwala, Mt Lavinia.Moratuwa,Panadura,Kalutara,Katukurunda,Nagoda, mathugama , Pelawatta , Neluwa )