Customer ability to filter inventory stock

I have our store set up to allow for customers to place back orders so I have the stock set to show even if out of stock. What I need is to create a module that will allow customer to be able to choose if they want to see all products or ones in stock only. I have looked and asked and there is no contribution or post that will allow this so here I am, looking for help.

I am more or less trying to duplicate (in a new file) the "product_listing_alpha_sorter" php file in the shop/includes/modules folder and named the new file "product_listing_stock_sorter". My problem is I need help editing the file to look for stock count and not create alpha characters. The drop down menu will appear to the right of the alpha sorter in the "tpl_index_product_list" file in my template.

Here is the file:

<?php
/**
 * product_listing_alpha_sorter module
 *
 * @package modules
 * @copyright Copyright 2003-2006 Zen Cart Development Team
 * @copyright Portions Copyright 2003 osCommerce
 * @license http://www.zen-cart.com/license/2_0.txt GNU Public License V2.0
 * @version $Id: product_listing_alpha_sorter.php 4330 2006-08-31 17:10:26Z ajeh $
 */
if (!defined('IS_ADMIN_FLAG')) {
  die('Illegal Access');
}

// build alpha sorter dropdown
  if (PRODUCT_LIST_ALPHA_SORTER == 'true') {
    if ((int)$_GET['alpha_filter_id'] == 0) {
      $letters_list[] = array('id' => '0', 'text' => TEXT_PRODUCTS_LISTING_ALPHA_SORTER_NAMES);
    } else {
      $letters_list[] = array('id' => '0', 'text' => TEXT_PRODUCTS_LISTING_ALPHA_SORTER_NAMES_RESET);
    }
    for ($i=65; $i<91; $i++) {
      $letters_list[] = array('id' => sprintf('%02d', $i), 'text' => chr($i) );
    }
    for ($i=48; $i<58; $i++) {
      $letters_list[] = array('id' => sprintf('%02d', $i), 'text' => chr($i) );
    }
    if (TEXT_PRODUCTS_LISTING_ALPHA_SORTER != '') {
      echo '<label class="inputLabel">' . TEXT_PRODUCTS_LISTING_ALPHA_SORTER . '</label>' . zen_draw_pull_down_menu('alpha_filter_id', $letters_list, (isset($_GET['alpha_filter_id']) ? $_GET['alpha_filter_id'] : ''), 'onchange="this.form.submit()"');
    } else {
      echo zen_draw_pull_down_menu('alpha_filter_id', $letters_list, (isset($_GET['alpha_filter_id']) ? $_GET['alpha_filter_id'] : ''), 'onchange="this.form.submit()"');
    }
  }
?>

Can someone help me with this?

This would make for a great contribution if we can come together as a community to build it.

Thanks,

Clint

From english.php I have added:

// extra product listing stock sorter	
  define('TEXT_PRODUCTS_LISTING_STOCK_SORTER', '');
  define('TEXT_PRODUCTS_LISTING_STOCK_SORTER_NAMES', 'Items starting with ...');
  define('TEXT_PRODUCTS_LISTING_STOCK_SORTER_NAMES_RESET', '-- Reset --');	

In tpl_index_product_list.php I have already listed:

  // draw stock sorter
  require(DIR_WS_MODULES . zen_get_module_directory(FILENAME_PRODUCT_LISTING_STOCK_SORTER));