You can use the --prefix option:
mkdir -p ./install/here/node_modules
npm install --prefix ./install/here <package>
The package(s) will then be installed in ./install/here/node_modules. The mkdir is needed since npm might otherwise choose an already existing node_modules directory higher up in the hierarchy. (See npm documentation on folders.)
In the documentation it's stated: Use the prefix option together with the global option:
The prefix config defaults to the location where node is installed. On most systems, this is /usr/local. On windows, this is the exact location of the node.exe binary. On Unix systems, it's one level up, since node is typically installed at {prefix}/bin/node rather than {prefix}/node.exe.
When the global flag is set, npm installs things into this prefix. When it is not set, it uses the root of the current package, or the current working directory if not in a package already.
(Emphasis by them)
So in your root directory you could install with
npm install --prefix <path/to/prefix_folder> -g
and it will install the node_modules folder into the folder
<path/to/prefix_folder>/lib/node_modules