Reference code
PQClean
PQClean is a project dedicated to collecting clean source codes of post-quantum cryptographic schemes under consideration by the NIST Post-Quantum Cryptography Standardization Project.The most reliable reference code of NTRU is PQClean repository (round3 tag). Note that the original archive submitted to the NIST PQC project does not include some of the parameter sets that were newly introduced during the review process.
Libraries
liboqs
liboqs is an open source C library for quantum-safe cryptographic algorithms.
You can find NTRU implementaion in the GitHub repository at the 0.7.2 tag.
Bouncy Castle
Bouncy Castle is a open-source cryptographic APIs for Java and C#.
You can find NTRU implementaion in both of Bouncy Castle for Java and Bouncy Castle for C#.Net .
Software code for the ARM Cortex-M4
pqm4
pqm4 is a collection of post-quantum cryptographic algorithms for the ARM Cortex-M4.
You can find NTRU implementaion in the GitHub repository at the Round3 tag.
Please note that the implementaion doesn't include NTRU-HPS-4096-1229 and NTRU-HRSS-1373.