I dunno. That assumes you can identify the ultimate source. If it's a local vintage dealer in Japan, you might need a contact there to physically visit the store. And even if the source sells online, their site may well be in Japanese (so if you don't speak the language, you'll need to find someone who does) and they may not have any experience with international shipping (so your purchase could get stuck in customs for a looong time). You'll have to establish a mutually acceptable payment method (local shops in Japan may not accept international credit cards). And if something goes wrong (e.g. the watch isn't as advertised), you have little recourse.
Buying from an established re-seller such as closer on a well-established platform with buyer protection (e.g. eBay or Chrono24) avoids those potential problems. Sure, you'll pay an additional fee for the convenience and protection, but it might be worth it to you. YMMV.
BTW, be sure to check both eBay and Chrono24. Many Japanese sellers list on both sites. The Chrono24 price is usually lower (since they charge less), but sometimes you can find special discounts and such on eBay.