According to here:
http://www.azarr.com/smith_power_pipes.htm
They can only be ordered direct:
[email protected]
270-554-7413
Jerry L Smith
3970 Contest Road
Paducah, KY 42001
But there is a better alternative... see here... these are all welded (Jerry's can uses JB Weld and it comes apart ... My exhaust tube came loose within 5 flights)
http://ohiomodelplanes.com/pipes/