If you are looking state or regional (rather than local transit) bus services, they are pretty rare...

Of course, chaining together local transit can be quite the adventure....


Since MegaBus's west coast operations went bust, the only non-Greyhound ("they don't call it 'floggging the dog' for nothing...") bus service in CA I'm aware of is Amtrak, California Shuttle Bus's service from LAX - SFO, The Chinatown bus service from Monteray Park to SFO Chinatown, and the hippie special Green Tortoise Express.

If you are looking for a non-auto/non-air way from San Diego to Los Angeles that isn't Greyhound....your real answer if probably Amtrak, or the rail commuter equivalent of the Coaster north to Oceanside, and change to Metrolink.