Renting a Car in LA

[Lazymeatball]

Has anyone ever done this before?
Has anyone ever picked up at one location and dropped off at another.
I'm going to be coming in to Union Station, want to find a place nearby to rent a car to get me to the Commerce, then to Santa Monica, then I want to drop it off at the airport and fly out of there.
So far the best location I've found is with Enterprise, but then I read something saying they don't specialize in one way rentals. Does picking up a car in downtown LA and then dropping it off at the airport count as one way. These websites are all completely useless so far.

Any help please.