Links and recommendations are in no specific order.
Steffens shop is nice and user friendly with loads of good items,quick responses and weelky updates.
Jim is a friendly chap with a very nice shop based in the Netherlands. Good items and frequent updates.
Stefans DNA sites should be known to all collectors of EKs & KVKs by now. A huge Database with countless variants of EKs & KVKs.
Militaria Berlin is one of the leading dealers worldwide. Great items added in weekly updates.
Check out Bastian's research blog, browse through different studies and researches on all kinds regarding militaria.