Backup solution for sharepoint online document libraries

It totally depends on what you want to do. What is it that you want to protect yourself from?

If you want backups like you would do in the on-premises environments then you would probably look at expensive 3rd party solutions.

In general I find that more and more customers simply don't bother.

Traditionally you would take backups for disaster recovery ( well this is Microsoft's problem as they manage the systems)

Then to handle the "oops I deleted a file", you can go to the recycle bin.

Then finally the option of "I made a mistake in my document update and I need to get my document back" ... You should have considered version control if you hadn't already done so.