Creator Dynamic Drop Down

Creator Dynamic Drop Down

Hi I am using creator to produce some site works on a particular project.
So have two forms, one with all our sites (Form Name: Sites) that contains around 500 site details, and another to book the actual works (Form Name: Service Call).
On the Service Form I have a lookup box (sites) that fetches Site Name form Sites, but looking to change this to a dropdown list box that uses deluge to populate the site names depend on a filter.
The reasoning behind this is, I only want populate with Site Names that haven't already been booked in the Service Form.
Any help would be appreciated.