Well the Specialized SIrrus comes in a XS size and is full riged but it is a 700c bike. What do you mean by "mountain"? Jamis Coda and Kona Dew are other full rigid 700c flat bar bikes.
http://www.specialized.com/bc/SBCBkModel.jsp?spid=12388
http://www.jamisbikes.com/usa/bikes/06_coda.html
http://www.konaworld.com/shopping_ca...6&parentid=253
Another option would be to buy the sprung bike and put a rigid fork on it...they are pretty cheap...but probably wont matck the pain job.
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