Hey there, Dominic. Welcome to planDisney! I love that you're looking to lock in the Disney magic by becoming a Magic Key holder, so let's see if we can get you sorted!
To utilize the Magic Key monthly payment plan for California residents, you'll want to make sure that you are signed in and that both your shipping and billing addresses on your MyDisney account have a California address before adding a 
Magic Key to your cart. Also, please note that the Imagine Key is only available to Southern California residents (living in zip codes 90000 to 93599), and your address must reflect it before you're able to purchase this Key. To check the address on your account. Head to the 
Disneyland Resort website and click the 
"My Disneyland" banner at the top right. From here, click on the "Manage MyDisney Account" button to open your current account information. If it is incorrect, go ahead and update it before attempting to purchase a Magic Key.
Dominic, currently, only the Inspire and Believe Magic Keys are on sale. The other two passes (Enchant and Imagine) are available for renewal only. Please be advised that all Magic Key sales could pause at any time, so it's best to get yours as soon as you can. Once you've locked in your Magic Key, be sure to check out the 
Magic Key Portal in the Disneyland app and follow the 
Disneyland Magic Key Instagram for all the fun Magic Key offerings, 
benefits, and news!
I hope this helps, Dominic. Please visit again if you have any other questions!
See you in the parks,
Melissa