How to load jquery ui autocomplete combobox?

Try to load the script in the footer section.

<script type="text/javascript"
        src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
        <script type="text/javascript"
        src="https://ajax.googleapis.com/ajax/libs/jqueryui/1.8.18/jquery-ui.min.js"></script>
<script type="text/javascript">
                $(document).ready(function(){
                    $("#clientAutocomplete").autocomplete({
                        source:'client_autocomplete.php',
                        minLength:1
                    });
                });
        </script>

This worked for me