Get your values using ajax method (you can use ajax method of jquery).
function BindListToCombo(cbo, list) {
$find(cbo).clearItems();
$.each(list, function (a, item) {
var comboItem = new Telerik.Web.UI.RadComboBoxItem();
comboItem.set_text(item.Name);
comboItem.set_value(item.Id);
$find(cbo).get_items().add(comboItem);
});
var comboItem = $find(cbo).findItemByValue(0);
comboItem.select();}
Use this function to bind the list to combo.