That depends on how those textboxes are being added/created.  If by 'not bound' you mean they are in Template columns you should be able to use .FindControl in one of the grid's events to grab that textbox. 
And again which event will depend on what is causing the postback to happen.
For the purpose of this code example I'll assume you are dealing with a Command item on the grid
Private Sub radGrid_ItemCommand(ByVal source As Object, ByVal e As Telerik.WebControls.GridCommandEventArgs) Handles RadGrid1.ItemCommand
  Select Case e.CommandName
    Case "Update"
      Dim txt as Textbox
      txt = e.Item.FindControl("textboxID")
      If Not txt is Nothing Then someObject.someString = txt.Text
    Case Else
      'do something else
End Sub
Hope that helps.