I'm not sure if this is the most efficient way to filter a drop down, but I would create a calculation field such as "OrgName_AgencyOnly" that looks like this: Case ( organizationType = "agency"; OrganizationName)
Then, use that calculation field to create the Value list for your drop-down.
Hope that helps.
You can also filter the value list through a relationship using the "use only values starting from..." option. How you set up your relationship would then define how the value list is filtered.
Found this example by searching this forum for "Filtered Value List"