how do i buy more ipad storage

It is impossible to increase the amount of RAM or internal storage in your iPad - as this is fixed during manufacture; what you have is fixed for the life of your device. 

You can, however, subscribe to a higher tier of iCloud storage - but this is not a substitute for internal storage.

About storage on your device and in iCloud – Apple Support

Manage your iCloud storage – Apple Support

If you have insufficient internal storage for your needs, you have two options:

• Manage the storage that you already have:

How to check the storage on your iPhone, iPad and iPod touch – Apple Support

• Replace your iPad with a model with adequate storage for your needs.

If you're talking about iPad Internal storage, there's no way to increase it. You need to make room to create som more space.

If you'r talking about iCloud storage, see this support article:--> What's the difference between device storage and iCloud storage? - Apple Support

Regards

Giulio

If you’re talking about physical storage on your iPad, you can’t. Your only option would be to either delete some of your data to free up space or purchase another iPad with a higher storage space. If you’re referring to more iCloud space you can do it in settings.

Any iOS device has an internal flash memory data storage drive of a fixed amount that cannot never, EVER be increased!

If you need additional internal iPad data storage there are ONLY two things to do.

Purchase a larger data storage capacity iPad OR backup, then manage/remove stuff off of your existing iPad.

If you need to free up free, internal data storage space on an iPad/iDevice, you need to start by backing up your iPad to either Apple's iCloud ( you may need to pay for additional, monthly storage space ) or by connecting your iDevice to a computer running the latest iTunes or, if a newer Mac with a much newer macOS, use the macOS Finder to do the device backup and/or some other offsite, “cloud” data storage/backup services, like Google Drive/Photos, BOX, DropBox, Amazon Cloud Drive, Flickr ( for photos/images ), etc.

Then, delete any installed apps that you no longer use or use infrequently enough that they aren't missed (you can always reinstalled any purchased or free apps at some time later, as you need them).

In addition, uninstall any games that appear to be using a sizable chunk of your iPad's internal storage, as well!

( A disclaimer to the immediate advice above. )

If you own an nearly 8-9-year old to nearly 10-year old iPad, you SHOULD ONLY delete apps that you have ever seldom used or no longer use as there is NO WAY now, from 2020 onward, to retrieve these old apps from the iOS App Store, any longer.

( Third party iOS app developers have permanently removed all of their older, legacy apps from Apple's iOS App Store servers forever!

There are, VIRTUALLY, NO more older, useful, popular, third party apps for over 8-10-year old iOS devices, any longer! )

Then, thin out/delete any music/videos/movies/TV shows/podcasts/screen recordings/photos-images/eBooks/

eMagazine/iBooks/audio books/Documents/PDFs, etc.

If you use a POP email account on your iPad, delete/thin out any old saved emails that maybe saved to your iPad.

To permanently delete photos/images off of your iDevice go into the Photos app and find the Recently Deleted folder/album and permanently delete any photos/images stored in that folder/album.

Also, you can check to see what other stuff is eating up all your valuable data storage space by going into the Settings App,,General Settings Panel, in the right hand column, tap Storage & iCloud usage panel ( in iOS 11 and later, it’s now called iPad Storage ), in the top Storage Panel, tap the Manage Storage panel, wait for a list of apps to generate that will tell you how much space each app is taking up on your iDevice.

Then, for older iPad models with a Home button, do a hard reset of your iPad by holding down both the Home and sleep/wake buttons simultaneously until your iPad goes to black and restarts with Apple logo, then release the buttons.

OR

For much newer, larger screen iPad models, without a Home button, try the new hard reset procedure for the new 2018/2020 iPad Pro and iPad Air 4 models.

1. Press and release the volume UP button.

2. Then immediately press and release the Volume DOWN button.

3. Then, press and hold the sleep/wake/power button at the top. You will see the Slide to Power off button, but keep on holding down the power button until you see the Apple logo, then let it go.

Once you have performed all the steps, wait for a few seconds and your 2018 iPad Pro will boot up completely to the iPad Pro’s Lock screen.

ALL iOS/iPadOS devices need to maintain, at the very least, a minimum of between 3-6 GBs of free data storage space, AT ALL TIMES, to insure normal/nominal functionalities/operation and general overall performance of any iOS device running iOS/iPadOS.

Good Luck to You!