How do I buy Apple Card online?

Hello ~ If you mean an Apple gift card …click on below…

Apple Gift Card - Apple

If something else we will need more details.

~Katana-San~

AppleCard is a credit card for U.S. residents and for which you must apply.

https://card.apple.com/apply/start

AppleCash is available to U.S. residents.

Set up Apple Cash - Apple Support

Apple Gift Card may be purchased online from Apple or at MANY retailers.

https://www.apple.com/shop/gift-cards