If the values in the dropdown are provided from a database then you are using some server-side code to get them so just use that code to sort the data before it writes it into the dropdown.
If you want to use javascript to do it client-side then your best bet is to put all of the options into a javascript array, use javascripts ability to sort an array then write the data out to the dropdown box either dynamically building the dropdown as the page loads or executing a function to populate the dropdown with an onload event.
If you do it client-side you have to know that your clients will not have javascript disabled though or you will not get the options in the dropdown.
Stamp out, eliminate and abolish redundancy!