How can I buy more storage ?

You cant buy more storage for your iPhone. If you want more iCloud storage click here

Manage your iCloud storage

Need more iCloud storage? You can easily make space or buy more storage.

When you set up iCloud, you automatically get 5GB of storage. You can use that space for your iCloud backups, to keep your photos and videos stored in iCloud Photos, and your documents up to date in iCloud Drive. You can make more space or buy more storage any time. 

https://support.apple.com/en-us/HT204247

Note: iCloud Storage does not increase what you have on iPhone.

Otherwise read here -> Manage your iCloud storage - Apple Support