While installing Cordova/Phonegap recently on a new laptop I ran into a 503 error during the process. Thankfully it was a simple enough fix and is the result of npm’s inability to access the default mirror.
Temporarily change the mirror,
<command> --registry http://registry.npmjs.eu/
Permanently change the mirror,
npm config set registry http://registry.npmjs.eu/
Below is a list of known mirrors you can use in-place of the above.