Getting Custom Post Type content from main-site of a Multisite

You could use the technique in this Stack Overflow answer, that’s basically:

switch_to_blog(1); //switch to main site
$latest_posts = get_posts($args);
foreach( $latest_posts as $post ) {
    // print post
}
restore_current_blog();

Just add 'post_type' => 'your-custom-post-type' to the query.