Is there a usable implementation of PBFT?
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