Can I purchase more storage

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.

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.

You can not buy extra storage on any device.