Verified answer

http://www.metro.net/ they will only have schudules for their buses

but

they also have a trip planenr which will help you plan a trip

but it will include or suggest another transit agency route if your interary calls for it

there also several other transit agencies operating in Los Angeles

in addition to metro the following have their own website and schedules

post to the net

http://www.bigbluebus.com/

http://www.ladotransit.com/

http://www.lbtransit.com/

http://www.octa.net/

http://www.foothilltransit.com/

mta.net

It doesn't necessarily cover every single bus system in the county, but it's a good start.

Google it :)