itune card

1.

See -> App Store & iTunes gift card: Apple Gift Card - Apple (UK)

• Add money to your Apple Account balance anytime, no gift card needed.

2.

or Where to buy a physical Apple Gift Card or App Store & iTunes gift card:

Actually, you can buy an Apple gift card at many places, it depends of course on where you live.

At an Apple Store or an Apple Authorized Reseller in your region certainly, but also at a good bookstore, supermarket and/or drugstore.

See also -> About Gift Card Scams -> About Gift Card Scams - Official Apple Support

3.

See -> How to redeem your Apple Gift Card or App Store & iTunes gift card

How to redeem your Apple Gift Card or App Store & iTunes gift card – Apple Support (UK)

When redeeming a gift card, (1) the amount will be credited to your Apple Account balance and (2) then you need to set your Apple Account balance as your default payment.

If you already use your Apple Account balance, the Apple Gift Card is automatically added to your Apple Account balance.

(1) See -> Check your Apple Account balance

Check your Apple Account balance – Apple Support (UK)

(2) See -> Add a payment method to your Apple ID

Add a payment method to your Apple ID – Apple Support (UK)

If you update your payment method or add funds to your Apple Account balance, It would take about 1 to 3 business days in order to process.

If you're still unable to update your payment method, Contact Apple Support.