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Here is a link to plans to build your own for free.
The instructions also have link at the end on how to build
your own mod for tuning the O2 sensor (or you can buy
them ready built).  He also sells them ready made for $220.

http://www.free-energy-info.co.uk/Smack.pdf

Their website is http://www.free-energy-info.co.uk/Smack.html

I've been reading up and thinking about building one myself.