How do I buy more storage on my iPad?

You cannot buy more storage for your iPad. It will always have the amount of space it came with. You can buy space in Apple's synching service, iCloud. Read the link in blue above your name on this page to understand the difference. The only way you can gain space is to delete photos, messages, apps, etc.. Be sure to save important photos and data to your computer or an external drive. Check this link...

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

It is impossible to increase the amount of 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.

If your internal storage is full, your only option is to better manage the storage that you have - and delete unused Apps or unneeded data. 

These support pages should prove to be helpful:

About storage on your device and in iCloud – Apple Support

What's the difference between device storage and iCloud storage? – Apple Support

Manage your iCloud storage – Apple Support

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

Clear Other storage on your iPhone or iPad - Apple Support

“Offloading” Apps can be a helpful method of creating temporary space for the software update.

https://9to5mac.com/2020/01/21/how-to-offload-apps-in-ios-to-save-space-without-deleting-their-data/

You cannot add storage to an iPad.