You could try using FontAwesome. It contains a sort-icon (http://fontawesome.io/icon/sort/).
To do so, you would
need to include fontawesome:
<link href="//maxcdn.bootstrapcdn.com/font-awesome/4.1.0/css/font-awesome.min.css" rel="stylesheet">
and then simply use the fontawesome-icon instead of the default-bootstrap-icons in your th
's:
<th><b>#</b> <i class="fa fa-fw fa-sort"></i></th>
Hope that helps.