Hi, Richard! I especially love your question because it shows you are planning for every detail - I like that in a person.
Walt Disney World theme park parking can be paid using cash, credit cards, debit cards, or
Disney gift cards. You can just pull up to any of the attended booths as you drive into the park of your choice, and the cast member will happily assist you with whatever form of those payments you choose to use.
While there are not pre-paid parking cards, you can purchase a Disney gift card for the same purpose. The gift cards can be purchased starting at $50 online, then in increments of $25 thereafter.
You can also visit just about any bank and purchase a pre-loaded Visa gift card for the amount you think you'll need for parking. The big reason to purchase the Disney gift card is because they are so darn cute, they become a souvenir after you use them. I have quite a few!
If you have any other questions while you plan your magical vacation, we are here and happy to help!
Anna