Looping through textboxes (0 to 5) in the code behind - or what container am I in to use FindControl?

I have several text boxes (TexBox0, Textboxl1, etc) that I would like to loop through in the code behind.  I was able to do this in a a simple web form using the FindControl 
Protected Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click    Dim tb1 As TextBox    For i As Integer = 0 To 1       tb1 = CType(Page.FindControl("TextBox" + i.ToString), TextBox)       MsgBox("TextBox =" + label1.Text)    Next End Sub
However when I use a web page with a master page it does not work.
First question is there another way to loop through controls in the code behind?
If not  is the best way to do it  to use  FindControl)  --- MS says "To access controls in a subordinate naming container, call the FindControl method of that container ...