This function repeatedly searches the string for the substring. It returns the index of the last occurrance it finds.

Public Function FindLast(ByVal source As String, ByVal _
    target As String, Optional start_at As Integer = 1) As _
    Integer
Dim pos As Integer

    ' Assume we won't find it.
    FindLast = 0

    ' Search for the target.
    pos = InStr(start_at, source, target)
    Do While pos > 0
        FindLast = pos
        pos = InStr(pos + 1, source, target)
    Loop
End Function