Update 12/08/24. It's been a few years since I made this video. Everything I say is still valid. Finding the Japanese Webike site can be tricky outside of Japan. Here's the address:
www.webike.net/sd/23331623/
The Buyee extension still works with this site.
You might consider buying a crash helmet directly from a Japanese website if you have a round head. I tested two examples from Arai and Shoei and the Japanese market version is more round than the one they officially export to the rest of the world.
In this video I show you how I imported helmets directly from Japan to the UK.