How can I buy more storage for my iPhone?

Keep in mind that you can't increase your phone's storage size, it is a fixed size.

Upgrade your iCloud storage plan -

https://support.apple.com/en-us/HT201318

Upgrade to iCloud+ - Apple Support

Read the above article subscribing to various levels of iCloud+

The short answer.....no.

But, you can buy more storage for your backups on iCloud.