If I understand you correctly, you already have your Main Form/Subform set up, and only need a Combobox to retrieve a Record in the Main Form, along with the related Subform Records. If this is correct, Add a Combobox to your Form’s Header, and when the Combobox Wizard pops up, select "Find a record based on the value I selected in my combobox," then follow the Wizard's instructions as you go down the Yellowbrick Road!
When you make a selection from the Combobox, the appropriate Main Form Record should appear, along with the related Records in the Subform.
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The problem with making anything foolproof...is that fools are so darn ingenious!
All posts/responses based on Access 2003/2007