Is there a usable implementation of PBFT? In the original Practical Byzantine Fault Tolerance (PBFT) paper they mention they implemented a fault tolerant NFS service. To do that they made a library for PBFT. Does that exist anywhere?

As that paper is quite old now and was well received, I thought it should not be hard to find implementations. However so far I only found incomplete and abandoned experiments.

RAFT has a list of implementations on their website. I am looking for something like this in the byzantine case.
Not necessarily NFS, but Hyperledger used PBFT for their blockchain platform.

This is a Go implementation: https://github.com/hyperledger-archives/fabric/tree/master/consensus/pbft

That is an old repo, they seem to be currently using a new repo with implementations in other languages as well, however I haven't gone through it in detail:
https://gerrit.hyperledger.org/r/#/admin/projects/

There are more related details in this question: https://stackoverflow.com/questions/41710738/pbft-algorithm-in-hyperledger

Good luck