That's called a moto collar. What's your price range? If it's 2,000 - 3,000 USD it'll be easy to find a nice one, if it's less you may want to consider looking online in places like y!jp and rakuten. If you're in Tokyo, you can check out Rick Owens, Undercover, Attachment, etc even if they're out of your price range to get a feel for what you'd want, and then maybe try to find one used online or go the made to measure route.