As of Jan 2014 (and earlier) the Southern Highway Buses run only from Maharagama . The fare is LKR 500. Journey is one and half hours (none rush hours) from Maharagama to Galle. It seems like the private buses have less air conditioning compared to the SLTB buses.
The worst public bus station toilet is in Maharagama. Much worse than Chilaw which is quite bad to start with. Surprised no one has complained.
Check with Pettah Central Bus Stand (+94) 11 2329604-5 about changes.
Info on Katunayake/Negombo to Pettah Highway service.