Puoi provare cosi.
codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>Untitled</title>
<script language="JavaScript" type="text/javascript">
<!--
function searchText(e) {
//--------- Stringa di ricerca
searchStr=document.getElementById('searchText').value;
if (e == null) {
if (event.keyCode != 8) {
searchStr = searchStr + String.fromCharCode(event.keyCode);
} // if (event.keyCode != 8)
} else {
if (e.which != 8) {
searchStr = searchStr + String.fromCharCode(e.which);
} else {
searchStr = searchStr.substr(0,searchStr.length-1);
} // if (e.which != 8)
} // if (window.event != null)
//--------- Svuoto le opzioni della select per il risultato
document.getElementById('mySelect').innerHTML = '';
//--------- Cerco solo se la stringa di ricerca non è vuota
if (searchStr != '') {
//--------- Select di destino
destSel = document.getElementById('mySelect');
//--------- Cerco nella select nascosta
oSelNas = document.getElementById('selNascosta');
numElem = oSelNas.length;
for (i=0;i<numElem;i++) {
var curOpt = oSelNas.options.item(i);
if (curOpt.value.substr(0,searchStr.length) == searchStr) {
var newOpt = document.createElement('OPTION');
ind = destSel.options.length;
destSel.options.add(newOpt,ind);
newOpt.value = curOpt.value;
newOpt.text = curOpt.text;
} // if (curOpt.value.substr(0,searchStr.length) == searchStr)
} // for (i=0;i<numElem;i++)
} // if (searchStr != '')
} // function searchText()
//-->
</script>
</head>
<body>
<select id="selNascosta" name="selNascosta" style="display:none">
<option value="aaaaa">aaaaa</option>
<option value="aabaa">aabaa</option>
<option value="aaaba">aaaba</option>
<option value="aaaab">aaaab</option>
<option value="baaaa">baaaa</option>
<option value="babaa">babaa</option>
<option value="baaba">baaba</option>
<option value="aaaac">aaaac</option>
<option value="babab">babab</option>
<option value="aaaad">aaaad</option>
<option value="baaad">baaad</option>
<option value="baaac">baaac</option>
<option value="caaaa">caaaa</option>
</select>
<table summary="">
<tr>
<td>Testo : </td>
<td><input type="text" id="searchText" size="25"></td>
</tr>
<tr>
<td>
</td>
<td>
<select id="mySelect" name="mySelect" size="10" style="width:100px">
</select>
</td>
</tr>
</table>
<script language="JavaScript" type="text/javascript">
<!--
//--------- Assegnazione dell'evento onkeypress del text box
document.getElementById('searchText').onkeypress = searchText;
//-->
</script>
</body>
</html>