The part number you are looking for is 105-000-151 (Brocade 2G USB Drive).  Not sure you can order this through DX though.  I've heard of people having success ordering it through Brokerage.
A Brocade USB drive is required in order for our Remote Support teams (RCM) to implement a switch/director upgrade. Is this part orderable in Direct Express and if so, does anyone know the direct express model number?  I have not been successful finding the part using the direct express product search engine.  Thank you.  
Can a Brocade USB Drive be ordered in Direct Express?