If the combo does not (cannot) contain the subform name, another way would be to write code that uses the combo value in its AfterUpdate event to set the source object property. If there are more than, say 3 or 4 possibilities, I'd probably use a Select Case block, e.g.
Code:
Select Case Me.cmbMyCombo
Case "apple"
set source to apple subform
Case "orange"
set source to orange subform
Case etc.
End Select
I'm presuming stuff here, like you know the syntax for the subform control reference and when to use or not use quotes around the Case values.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.