wp-next-prev-function.php

// Handy function to get next/prev post of any post type, based on any ordering criteria

function fq_get_nearest_posts( $post_type=null, $orderby='menu_order', $order='ASC' ) {

	$args = array(
		'post_type' => $post_type ? $post_type : get_post_type(),
		'orderby' => $orderby,
		'order' => $order,
		'posts_per_page' => -1,
	);
	$q = new WP_Query( $args );
	$keys = array();
	$posts = array();
	while ( $q->have_posts() ) : $q->the_post();
		$keys[] = $q->post->ID;
		$posts[] = $q->post;
	endwhile;
	wp_reset_postdata();

	// Identify the position of the current product within the $posts-array 
	$current = array_search( get_the_ID(), $keys );

	// Identify ID of previous product
	$prev = $posts[$current-1];
	if( !$prev ) {
		$p = $posts;
		$prev = array_pop( $p );
	}
	// Identify ID of next product
	$next = $posts[$current+1];
	if( !$next ) {
		$n = $posts;
		$next = array_shift( $n );
	}
	
	return array($prev,$next);		
}