Get the Previous / Next Post link by Custom-Field on WP.

get_nextprev_post_link_by_cf.php

<?php
function get_prev_post_cf($link="&laquo; %link", $title="%title", $post_type="post", $meta_key, $meta_value, $label) {
	global $wpdb, $post;
	$prev = $wpdb->get_row($wpdb->prepare("
		SELECT ID, post_title
		FROM $wpdb->posts p
		LEFT OUTER JOIN $wpdb->postmeta pm on p.ID = pm.post_id
		WHERE post_type = '{$post_type}'
		AND post_status = 'publish'
		AND post_date < '".$post->post_date."'
		AND meta_key = '{$meta_key}'
		AND meta_value = '{$meta_value}'
		ORDER BY post_date
		DESC LIMIT 1;
	"));
	if($prev) {
	//	$title = preg_replace('/%title/',$prev->post_title, $title);
		echo '<li class="prev"><a href="' . get_permalink($prev->ID) . '" rel="prev">' . $label . '</a></li>';
	}
}
function get_next_post_cf($link="%link &raquo;", $title="%title", $post_type="post", $meta_key, $meta_value, $label) {
	global $wpdb, $post;
	$next = $wpdb->get_row($wpdb->prepare("
		SELECT ID, post_title
		FROM $wpdb->posts p
		LEFT OUTER JOIN $wpdb->postmeta pm on p.ID = pm.post_id
		WHERE post_type = '{$post_type}'
		AND post_status = 'publish'
		AND post_date > '".$post->post_date."'
		AND meta_key = '{$meta_key}'
		AND meta_value = '{$meta_value}'
		ORDER BY post_date
		DESC LIMIT 1;
	"));
	if($next) {
	//	$title = preg_replace('/%title/',$prev->post_title, $title);
		echo '<li class="next"><a href="'.get_permalink($next->ID).'" rel="next">' . $label . '</a></li>';
	}
}
?>

<ul>
<?php
	get_prev_post_cf('', '', 'post_type', 'customfileld_key', 'customfileld_value', '&lt; Prev');
	get_next_post_cf('', '', 'post_type', 'customfileld_key', 'customfileld_value', 'Next &gt;');
?>
</ul>