[Vampir3]

<select id="option">
	<option>first</option>
	<option>second</option>
	<option>third</option>
</select>

<button class="previous">&#8592</button>
<button class="next">&#8594</button>

<script>
window.onload = function () {
	var sel = document.body.children[0];
	document.body.onclick = function (e) {
		e = e || event;
		var target = e.target || e.srcElement;
		if (target.className == 'previous') {
			if (sel.selectedIndex != 0) {
				sel.selectedIndex--;
			}
		} else 
		if (target.className == 'next') {
			if (sel.selectedIndex != sel.children.length - 1) {
				sel.selectedIndex++;
			}
		} 
		
	}
}
</script>

спасибо, всё отлично работает