How to bind a grid filter dropdownlist control to a datasource in code?

I have a filter template defined within an Obout grid that contains a dropdownlist. I want to set the datasource in code behind on page load. How do I use findcontrol to find the control and set the datasource? I tried something like this.... but returns null.

DropDownList ddl = new DropDownList();

ddl = (DropDownList)(GridOrderNotes.FindControl("ddlNoteCatFilterOrderNotes"));
...