<<Disclaimer: Verify this information before applying it to your situation.>>
 
Potato starch can be found in any supermarket that sells any "Jewish"
products.  It'd likely be with mixes for matzoh balls, potato pancakes
etc.  I suppose some stores only carry these things at Passover (spring)
but my store carries it year round. And there aren't that many Jews in
Pennington, NJ!
LL