Verdaccio [https://github.com/verdaccio/verdaccio https://github.com/verdaccio/verdaccio] looks like a viable open source alternative. Presumably you could use

Verdaccio [https://github.com/verdaccio/verdaccio https://github.com/verdaccio/verdaccio] looks like a viable open source alternative. Presumably you could use it to download the packages you needed, then disable proxying to the NPM registry.