Where to find the PHP pages for the new Custom post types?

If you've used a plugin to create a custom post type through the WordPress admin interface, my initial assumption would be that it stored the information for that custom post type in the database itself instead of creating a PHP file with the required declaration in it. I would highly recommend that you never create custom post types by using a plugin to do it for you.

The required PHP code to register custom post types is not difficult to understand and can be easily added to a theme (even though I wouldn't recommend attaching a custom post type to a theme's files either). The most ideal way to register and manage custom post types for a project would be to create a tiny plugin specific to that project. This way, the custom post type is not only accessible when the specific theme it is attached to. Themes should be for the presentation of a website's data and plugin's should be for the management of it.

Below is a very simple one-file plugin that you could create to manage your custom post types:

<?php
/*
Plugin Name: Project Plugin
Plugin URI: http://example.com
Description: Example description for a project-specific plugin
Version: 1.0.1
Author: Author_Name
Author URI: http://example.com
*/

// Create the map post type on WordPress initialization
add_action('init', 'create_map_post_type');
function create_map_post_type() {
    register_post_type('map',
         array(
            'labels' => array(
                'name'                  => 'Maps',
                'singular_name'         => 'Map',
                'add_new'               => 'Add New',
                'add_new_item'          => 'Add New Map',
                'edit_item'             => 'Edit Map',
                'new_item'              => 'New Map',
                'all_items'             => 'All Maps',
                'view_item'             => 'View Map',
                'search_items'          => 'Search Maps',
                'not_found'             => 'No Maps found',
                'not_found_in_trash'    => 'No Maps found in Trash',
                'parent_item_colon'     => '',
                'menu_name'             => 'Maps'
            ),
            'public' => true,
        )
    );
}

All you'd need to do is drop that code into nameofplugin.php and put that into a nameofplugin folder in the wp-content/plugins folder, and boom, you've just created a plugin that creates the custom post type 'map'.

Refer to the documentation for register_post_type() to learn more about the other arguments you can pass to it to configure your custom post type appropriately.