I’m trying to find the balance of an Apple Gift Card

This support article tells you how to find the balance of an Apple Gift Card or an App Store & iTunes gift card; click here --> 'Check your gift card balance' --> Check your gift card balance - Apple Support

To check the balance of your Apple Gift Card, follow this link --> https://secure.store.apple.com/shop/giftcard/balance

You can't check the remaining balance on an App Store & iTunes Gift Card. The balance on an iTunes or App Store gift card itself is either what it says on the face of the card, or nothing because it has already been redeemed. The only way to check is to try redeeming the card which of course will transfer the entire value, if any, to your account.

Redeem your App Store & iTunes Gift Card - How to redeem your Apple Gift Card or App Store & iTunes gift card - Apple Support

Then check your Apple Account balance as described in this document -->

Check your Apple Account balance - Apple Support

Note that there is no need to set your Apple Account balance as your default payment. It is always your default payment method unless a purchase cannot be made that way. You couldn't change that even if you wanted to.

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, the amount will be credited to your (1) Apple Account balance and (2) you need to set your Apple Account balance as your default payment.

(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)

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If you have a valid payment method on file, instead of buying or order an App Store & iTunes gift card, you can Add money to your Apple Account balance anytime, no gift card needed, see -> Apple Gift Card - Apple (UK)

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, at an Apple Store or an Apple Authorized Reseller in your region.

See alo this Apple article:  If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card

If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card - Apple Support

Beware of -> About Gift Card Scams -> About Gift Card Scams - Official Apple Support 

If you still need help, get support from Apple directly -> Get Support or call Apple in your region see -> Contact Apple for support and service or check -> here for more support and assistance.