As you appear to aware, if purchased online from Apple and associated with your AppleID account, your past purchases are usually accessible - along with electronic copy invoices - from your account.
If the copy invoice isn’t accessible, your only option is to seek assistance directly from Apple Support.
You can contact the Support Team using the Support link at top-right or bottom-left of this page.
https://getsupport.apple.com/
Alternatively and perhaps easier, you can initiate and manage your support cases using the excellent Apple Support App. If not already installed on your iPad, the App can be downloaded from the App Store:
https://apps.apple.com/gb/app/apple-support/id1130498044