$(document).ready(function () {
        $("#Countries").select2({
            placeholder: "Выберите страну",
            allowClear: true,
            ajax: {
                url: '/JData/CountriesAutocomlete',
                dataType: 'json',
                type: "POST",
                data: function (term) { return { term: term }; },
                results: function (data) {
                    return {
                        results: $.map(data, function (item)
                        { return { text: item.CountryName_ru, id: item.id } })
                    };
                }
            }
        });

<input type="text" id="Countries" style="width:300px" />

<link href="http://ivaynberg.github.com/select2/select2-3.4.6/select2.css" rel="stylesheet" type="text/css" />
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js"></script>
<script src="http://ivaynberg.github.com/select2/select2-3.4.6/select2.js"></script>