Where do I purchase more space for my ipad

Nowhere. Physical storage space on the iPad cannot be increased in any way,

You can only purchase iCloud storage space, which is space for backups, photos and other files on an Apple server somewhere, and will not change the lack of storage space directly on your iPad in any way.

You either delete things on your iPad to make more space, or buy an iPad with a larger storage capacity.

You don’t.

All iOS devices have a fixed amount of internal flash memory data storage space.It can never be increased.

If you need a larger internal data storage iPad, then you must purchase a larger internal data storage iPad model!

If you need to free up free data storage space on an iPad/iDevice, you need to start by backing up your iPad to either Apple's iCloud or by connecting your iDevice to a computer running the latest iTunes and/or some other offsite, “cloud” data storage/backup services, like Google Drive/Photos, DropBox, BOX, 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).

Then, thin out/delete any music/videos/movies/TV shows/podcasts/screen recordings/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, it’s 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, 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.

iOS devices and iOS/iPadOS need to maintain, at the very least, a minimum of between 2-3 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!

You can’t. When you purchase your iPad, the space that is described on the package is what you are getting. No more.

An iPad’s space may never be increased.

You may trade your iPad for a more spacious model.