AFAIK, you can't. For one thing, the Form Header, Form Detail and Form
Footer sections each have their own tab order. In other words, on a single
form, you can have 3 separate controls that have TabIndex = 0.

If you wanted to live with that restriction, you could loop through all of
the controls on the form, looking for the one that has that value for its
TabIndex. Note that not all controls support the TabIndex property.

In addition to Doug's comments, you really do need to know
the control's section.  Then  you can loop through the
section's Controls collection.  E.g. if the control is in
the detail section:

    On Error Resume Next
    For Each ctl in Me.Setions(acDetail).Controls
        ndx = ctl.TabIndex
        If Err.Number = 0 Then
            If ndx = x Then
                'do something
            End If
        End If
    Next ctl