This is the basic singleton implementation. (I'm giving the link because, if you are going to look on the internet to find a copy-pastable implementation of it, you might find a poor one (like the one from AndySpb) and screw your code up with it, so at least, this link has a proper implementation (but only a very basic one, i.e. no thread-safety and no cross-modularity or ABI)).
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 ...