You can also try Finisher's & Auto Modeli G.T in Ueno. Their address is at the bottom of the page.
http://finishers.jp/
In Yokohama's Ramen Museum, there is a slot car place with a whole wall of discontinued kits. The kits are for sale, ask the shopkeeper to let you in for a look. Don't expect too much for the ramen, though.
http://www.raumen.co.jp/english/