You might start with the ICAN (International Cesarean Awareness Network) 
web site.  They have a good resource list.  Additionally, check out The Farm 
statistics from Ina May Gaskin, and even the American College of Obstetrics 
and Gynecology.  The ACOG is not known to the be an organization that is 
particularly friendly to women wishing for vaginal birth, but even they suggest 
a trial of labor for women with a previous section.  Of course, after threee, 
they may not, but it certainly addresses some of the weaknesses in the 
arguments of this mother's current provider in recommending an automatic 
section for births 2 and 3.  

And, if you are feeling squirrelly and want some anecdotal evidence, Google or 
search on YouTube for HBA4C.  There is a pretty moving story about a woman 
who had a homebirth after four C-sections and had no complications 
whatsoever.  Again, anecdotal, and not a guarantee of anything, but I'd say it 
would certainly inspire someone to research it more thoroughly after they 
realize something like that is possible.  

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