My last snippet works for integers from 1 to MaxInt. Try it!
This snippet just tells you how it could be done. How it should be done is entirly up to you to decide.
Another possibility is the use of the SubString method and to concatenate all the substrings together with ",".
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 ...