The most recent release of dcm2niix is here:
  https://github.com/rordenlab/dcm2niix/releases
Recent stable releases are also included with MRIcroGL. You can get that at one of these two sites:
  https://github.com/neurolabusc/MRIcroGL/releases
  https://www.nitrc.org/projects/mricrogl/
Some users have reported problems downloading from the NITRC site, but this seems to be specific to their browser as I have not been able to replicate. If you are having problems you may want to try a different browser for the download.