Show Custom Field of Current Page in For Loop

You must pass the $post->ID to the function or setting up the current post via a call to
$specialPosts->the_post() that fills it for you

See get_post_custom_values documentation in the codex or this page on how to use it.

Try

<?php 
            $specialPosts = new WP_Query();
            $specialPosts->query('pagename=page-'.$i.'');

            while($specialPosts->have_posts()): $specialPosts->the_post();

               $mykey_values = get_post_custom_values('custom_select');
                   foreach ( $mykey_values as $key => $value ) {

            ...
            endwhile;