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