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>