Best practice: 7,000 item list lookup

We have multiple fields on a list to populate a charge code scenario below: 

1. First the user selects the Department they are ordering from 
2. Then a drop down appears for them to select a Charge Code Category associated with the Department
3. From there another drop down appears for them to selected an associated Charge Code Description 
4. After those 3 are selected the actual CHARGE CODE should appear for what they are purchasing 

The problem is the charge code list has about 7,200 items.   What is the best way to achieve our goal here?  

THANK YOU IN ADVANCE!