The USDOT # is available free of charge and is easy to obtain via telephone or online. 

The phone number to call is 1-888-464-8736.   The hours of operation are Monday - Friday 8:30 am - 12:00 noon and 1:00 pm - 4:30 pm. 

This web site has step-by-step instructions in obtaining a USDOT number or updating an existing registration:

This service involves no fee and provides step-by-step explanation and guidance to complete registration.  Upon completion of all requested information, carriers will be issued their unique USDOT number while on line.  The screen displaying the assigned USDOT number should be printed and a copy retained in your file.  A USDOT notification letter will be sent to you by mail.