Honestly, those pieces may be hard to come by. However, I am a size small and most of the good stuff second hand is medium or large so hopefully easier for you to find! I recommend Yahoo Japan or Rakuten Marketplace in Japan. You may be able to find them for sale but at a premium.
I've been looking at some items (not just RRL, and using a proxy as those website are only in Japanese, right?) but never actually bought anything. The pricing on many seems reasonable, but I'm hesitant about shipping and other additional costs. Got any advice?