I've never bought online. If you are going to LAX, the ticket can be bought on site before boarding. If you are flying into LAX, you can board the bus without ticket. Then, you will have to buy ticket at the station, at one of the kiosks, or from attendant, upon arrival, in order to be able to exit the station.
You can either purchase one online at their website of buy one when you get there. Its probably a better idea to buy one there because the times are more accurate at their kiosks.
You really don't need to, they have kisoks at the terminal.
Visit its website
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