Get post by custom field

// WP_Query arguments
$args = array (
	'post_type'              => 'foo',
	'posts_per_page'         => '1',
	'meta_query'             => array(
		array(
			'key'       => 'bar',
			'value'     => 'baz',
		),
	),
);

// The Query
$query = new WP_Query( $args );