Fast pass is free and included with your tickets. To book it online you will have to buy the Magic Pass after downloading the disney app. You should be inside the park to access it.
There are no fast passes for sale online. Fast passes are only available at Disneyland for free. If you want to buy a regular ticket, you can add the max pass to it when you but the ticket, but only can use it once inside.
There's a Disneyland app on the app store or Google play store and you do it through that
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