Qualcosa del genere ...

Nelle dichiarazioni del form

codice:
Private Declare Function SendMessage _
    Lib "user32" Alias "SendMessageA" _
    (ByVal hwnd As Long, ByVal wMsg As Long, ByVal wParam As Long, ByVal lParam As String) As Long

Private Const CB_FINDSTRINGEXACT = &H158
e dove ti serve

codice:
    Dim fnd As Long
    fnd = SendMessage(Combo1.hwnd, CB_FINDSTRINGEXACT, 0, "Stringa da cercare")
    If fnd = -1 Then
        MsgBox "Elemento non trovato"
    Else
        Combo1.ListIndex = fnd
    End If