I've bought S-Trax from Sidetracks (http://www.sidetracks.net/Products/ManByProdID722.htm). He specializes in S-Gauge.
I've also bought S-Trax from C. T. McCormick (http://www.cttrains.com/mth7.htm). If it's a small order he may have to order it from a middle-man and it will take longer.
Both dealers sell the track at less than retail.