How to build a cheap device to determine where Wi-Fi interference is coming from?
The WiSpy is based on a Chipcon (now TI) CC2500 chip, which is a general-purpose 2.4GHz digital radio.  By scanning through the channels, it can be turned into a spectrum analyzer. http://tim.cexx.org/?p=646

So you need a board with a CC2500 radio.   Search ebay for "CC2500" and you can find them for http://www.ebay.com/itm//260900652131

Here is a similar project with a Chipcon chip at a different frequency:
http://ossmann.blogspot.com/2010/03/16-pocket-spectrum-analyzer.html