how do I move ios apps that doesn't exist in appstore

You can't copy apps from a device that's on iOS 9+, that version of iOS introduced a developer feature called app thinning : If your apps aren't transferred from iTunes to your device after updating to iOS 9 - Apple Support . So unless you already have a copy of the apps on your computer or backup of your computer, and if the developers doesn't support the downloading of older compatible versions of their apps, you won't be able to get them on the old iPad

You don't have copies of them on your computer and/or on your backup of your downloads/computer ? If not, and if the device that they are on isn't on iOS 9+ and if you haven't updated to iTunes 12.7+, you should be able to copy them from the device over to your computer's iTunes library via File > Devices > Transfer Purchases and you can then sync them to the old iPad.