WebBrowser1.Document.Window.Frames returns an array containing all the frames of a web page. You can get the frame you are interested in by using its position in the array.

Code:
        'see how many frames are there
        MessageBox.Show(WebBrowser1.Document.Window.Frames().Count.ToString)


        'show the source code of each frame
        For i As Integer = 0 To WebBrowser1.Document.Window.Frames.Count - 1
            MessageBox.Show("Frame number: " & i & vbCrLf & vbCrLf & WebBrowser1.Document.Window.Frames(i).Document.Body.OuterHtml)
        Next