When I purchased the piece a few years ago, I emailed the publisher Atelier M and ordered it direct from them. Here is their email address: info@atelierm.net
They sent a paypal invoice once I confirmed my shipping address. I think it came out to about $68 USD, which included EMS shipping from Japan to the US.
Here is the title and composer in Japanese, which might be helpful to include in your email, just to make sure they know which piece you're requesting to purchase!
Toshio Mashima (真島俊夫) - Deux Rêves (2つの夢)
Willson 2900 TA-1 Euphonium - Denis Wick 4AM
Yamaha YSL-643 Trombone - Bob Reeves BrassArk 5G "Gladstone"
Yamaha YSL-8440 Trombone - Denis Wick 5BS
VMI 3301S BBb Tuba - Schilke Helleberg
Past:
York Preference 3067 Euphonium - Denis Wick 4AL
Benge 165F Trombone - Benge Marcellus
Wessex BR140 Baritone - Denis Wick 6BS
F.E. Olds Special Trombone (ca. 1941)