It should still be available for purchase, they released the truck as the Blackfoot Xtreme not that long ago. The transmission parts for the Frog and Subaru Brat should be the same as well.
I do not know if this eShop is reputable, but they have Tamiya parts...
http://www.etamiya.com/shop/tamiya-5...08-p-4090.html
You should be able to find them on eBay too...