You can buy them at the entrance or online. The advantage of getting the tickets in advance is that you can get some Fastpasses prior to arrival. The Fastpasses will get into a shorter line and reduce your wait time. Check the Disney web page for more information.
No you can buy in place.
Nowadays you need a park pass reservation to get into the park, so wherever you get the ticket you need to link it to the my Disney Experience site or app so you can make a reservation to get into the park.
Go through undercovertourist.com
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