The authors of the original NTM paper did not publish their source code
The first stable open-source implementation was published in 2018 at the 27th International Conference on Artificial Neural Networks, receiving a best-paper award. 
Other open source implementations of NTMs exist but are not sufficiently stable for production use.
The developers either report that the gradients
of their implementation sometimes become NaN
during training for unknown reasons and cause training to fail;
report slow convergence;
or do not report the speed of learning of their implementation.