I have purchased Iraqi currency ( the new kind). It is scheduled to
become a globally recognized currency oon July 1st ( i think ). Here
is the question:
   
  Where in the United States can I exchange very large ammounts of
foreign currency (hypothetically $500,000 worth )?

  And at what percentage will this be taxed when it is converted?  

  And what can I do legally to mitigate some of these taxes?

You might be able to make more money selling it on eBay than actually exchanging it.