You need to purchase them from Disney, either at the gate or through disneyland.com. However, I believe they have a current pause on any new ticket or annual pass sales.
As said above, sales of new annual passes (along with all new ticket sales and even AP renewals) have been temporarily paused so you're unlikely to find any information on Disneyland's website.
However, Flex, Deluxe, Signature, and Signature+ were the four Disneyland AP levels available to everyone prior to the sales pause. We do not know when sales will resume or if offerings will be the same. When/if they resume and if they resume as they were previously offered, then those are the four you would have access to purchase and use.
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