What you'll need to build your own supercomputer:
Depending on the amount of nodes you have, and your router/modem, you may also need to buy a switch.
You may also want to opt for some stack-able Raspberry Pi cases if you have many nodes.
The switches I use: http://www.newegg.com/Product/Product.aspx?Item=N82E16833156250
The stack-able case I use: https://www.etsy.com/listing/162751021/dual-stackable-media-center-case-for?ref=related-0