Combo Box to display ID's used only

I am trying to create a combo box that will only display the ID's that have been used on the master table. I have one table called customer type that gives a unique key and description for customer types. When the master table is populated with customers the users choose from a drop down list that is populated with the unique ID and description from the Customer Type table.

Now I am in the process of creating a multi-parameter query that will display customers based on what the user chooses from multiple combo boxes on a form. The challenge I am running into is there are some customer types that have not been used yet. I need that customer type combo box to only show the ones that have been used on the master table, not all of them from the Customer Type table.

I have searched and searched this board, in fact I have spent the last couple hours doing so, trying to find my answer and keep coming up empty. It is probably in how I am doing my search of the board, but I have tried several combinations of words and still cannot seem to find the answer. I know it is probably something simple but it is costing me a lot of time to find it.

Thanks,