If you did not receive a copy of the authorization letter and you are a member of the NPGA you should contact your association staff for a copy or find it online at the Hazmat Safety Homepage at:
http://hazmat.dot.gov/sp_app/special_permits/docs/13000/SP13341.pdf
If you are not a member of NPGA your authorization letter should have been sent to you. If not, you should contact the Office of Hazardous Materials Special Permits and Approvals at 202-366-4535.