For locals who can go to the parks tomorrow, the easiest way to purchase a MK is to enter the park and upgrade whatever ticket you used to get into the park.
In January, the in-person ticket booth MK queue was over seven hours . There were people who wanted to upgrade their So Cal ticket to an annual pass. CM told them to make a reservation for one of the parks, walk inside the gate then go on their app to upgrade. For the people who just came in person to purchase a MK, they told them to purchase a single day ticket on their phone, make a reservation for that day, go into the parks and upgrade from their app. If your ticket is active (ie. you used it to enter the park), it takes seconds to upgrade to a MK. The value of your ticket is applied towards the MK.
The only caveat is that your MK becomes active on that day and expires one year later.
ETA: If you are going in person, be careful about using the DTD parking lot. Most people thought they would be there for a couple of hours. No one thought it would take that long and that they would have to pay a small fortune for parking.