The Roxas and Xion music boxes that were recently available at the Kingdom Hearts Orchestra - World of Tres- in Yokohama are being sold by Aitaikuji for ¥7,400 ($67.34) each!
Thanks to Churro for the tip!
UPDATE [Dec 31, 2019]: Instagram user knzo025 was kind enough to give us close up pictures of the music boxes from the recent concert in Yokohama. You can view them in the gallery below.
UPDATE [Jan 20, 2020]: The Other Promise and Vector to the Heavens music boxes can be pre-ordered on Square Enix E-Store. Each one will cost ¥7,480 (about $68.00) and will release January 31st. You can pre-order The Other Promise here and Vector to the Heavens here.
Thanks to @aibo_ac7 for the update!