You can specify the starting index in the Substring method. Let's say you are looking for the string "the" in the phrase "the quick brown fox jumped over the lazy white dog". Once you have found the index of the first "the" you can start looking at that index plus onbe to search for the next occurrence. Or you can start looking at that index plus the length of the string you are looking for.
For Each ctrl As Control In Me.Controls("pnlMainPanel").Controls
If ctrl.GetType Is GetType(System.Windows.Forms.Panel) Then
For Each subCtrl As Control In ctrl.Controls
If subCtrl.GetType Is GetType(System.Windows.Forms.TextBox) Then
If subCtrl.GetType Is ...