Apenas repositório mesmo:

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
<script type="text/javascript">
jQuery(document).ready(function(){
	var combo = jQuery("select[name='combo']");
	
	
	jQuery("input[name='selecionar']").click(function(){
		var button = jQuery( this );
		combo.find("option").each(function(){
			if( jQuery( this ).text()==button.val() )
			{
				combo.val( jQuery( this ).val() );		
			}
		});	
	});	

});
</script>

<select name="combo">
	<option value="2">Selecione</option>
	<option value="1">Item 1</option>
	<option value="2">Item 2</option>
	<option value="3">Item 3</option>
</select>

<input type="button" name="selecionar" value="Item 1" />
<input type="button" name="selecionar" value="Item 2" />
<input type="button" name="selecionar" value="Item 3" />