How do you buy more storage

If you're referring to the iPad's internal storage, it's not upgradable.

If you're referring to iCloud storage, click here and follow the instructions. The iCloud+ service is a rental, not a purchase, and will be billed monthly.

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As Niel stated, you may not purchase more storage for an iPad.

However, you may purchase an iPad with more storage.

Apple Trade In - Apple

iPad - Apple