Generic autoloader for classes named in WordPress coding style.

class-autoload-wp.php

<?php

if ( ! class_exists( 'Autoload_WP' ) ) {
	/**
	 * Generic autoloader for classes named in WordPress coding style.
	 */
	class Autoload_WP {

		public $dir = __DIR__;

		function __construct( $dir = '' ) {

			if ( ! empty( $dir ) )
				$this->dir = $dir;

			spl_autoload_register( array( $this, 'spl_autoload_register' ) );
		}

		function spl_autoload_register( $class_name ) {

			$class_path = $this->dir . '/class-' . strtolower( str_replace( '_', '-', $class_name ) ) . '.php';

			if ( file_exists( $class_path ) )
				include $class_path;
		}
	}
}

You can append the sub folder name that you need in the constructor, to the dir property. And choosing to have your own autoloading class with a different name.