Multiple Drop-Down custom taxonomies search

WordPress in version prior to 3.1 does not support querying multiple taxonomies, you will need to install Scribu’s plugin Query Multiple Taxonomies to fix that.
and in order to get the dropdown select box you can use wp_dropdown_categories()
like this:

$args = array(
    'taxonomy' => 'location',
    'hide_empty' => 0,
    'echo' => 1,
    'name' => 'location'
);
if (isset($_GET['location'])){
    $args['selected'] = intval($_GET['location']);
}
wp_dropdown_categories( $args );