Filtered dropdown field for Dynamics CRM 4

we would like to add a new field to our Opportunity form. The field will allow users to choose from a two level category list. The list will be populated from data like this... Parent Item 1 Child Item 1.1 Child Item 1.2 Parent Item 2 Child Item 2.1 Child Item 2.2 Child Item 2.3 The field would comprise of two dropdowns. The user chooses Parent Item in the first field. The second dropdown will then show filtered results based on the parent item. we need someone to develop this control and assign it to a new field on our Opportunity type. We are running Dynamics CRM 4