There are many places that sell wide band lambda sensors.
My prefered ones because they work as well as high end ones are the one that was developed from a DIY board and is sold by Tech Edge:
http://www.techedge.com.au/vehicle/wbo2/default.htm
And the new one from Innovate that just won the best new product at SEMA:
http://www.innovatemotorsports.com/
Innovate has a mailing list on their product and would be a good place to ask questions about AFR and lambda readings and how to use them as a tuning tool.
http://autos.groups.yahoo.com/group/InnovateLM1/