You can use this function, linq might be a more elegant way.
Dim allTxt As New List(Of Control)
For Each txt As TextBox In FindControlRecursive(allTxt, Me, GetType(TextBox))
'....'
Next
Public Shared Function FindControlRecursive(ByVal list As List(Of Control), ByVal parent As Control, ByVal ctrlType As System.Type) As List(Of Control)
If parent Is Nothing Then Return list
If parent.GetType Is ctrlType Then
list.Add(parent)
End If
For Each child As Control In parent.Controls
FindControlRecursive(list, child, ctrlType)
Next
Return list
End Function