The HN "basic" source code is available online[1] or if you're looking for something modern, try Telescope which is built in Node and the Meteor framework[2]. There is also a demo available[3] Or you could try forking the Reddit source code[4] and setting it up, but im not sure if you'll need multiple subreddits functionality.

[1]https://github.com/wting/hackernews

[2]http://telesc.pe/

[3]http://demo.telesc.pe/

[4]https://github.com/reddit/reddit