Data Validation List Function

This UDF should do what you want.

Function ValidationListSource(validatedCell As Range) As Variant
    Set validatedCell = validatedCell.Cells(1, 1)
    On Error GoTo NAError
    With validatedCell.Validation
        If .Type = 3 Then
            ValidationListSource = Mid(.Formula1, 2)
        Else
NAError:
            ValidationListSource = CVErr(xlErrNA)
        End If
    End With
    On Error GoTo 0
End Function