Dynamic Drop Down on Search Page

I would like to see dynamic drop down functionality added to Search pages.

I'm using Knack to store information about continuing education credits for professional development courses, and I have a Search page on my site that allows visitors to find upcoming classes by Credit Type. Some Credit Types have multiple Credit Categories associated with them. For example, when a visitor selects a Credit Type of "NJ / CT Pesticide Applicator Credits" I would like the Credit Categories drop down to automatically only display those categories associated with the NJ / CT Pesticide Applicator Credits credit type.

My webpage with the Knack search embedded is here: http://www.cpe.rutgers.edu/credits/