I had to find it via a guide by another forum member:
Start a new world, use the seed "Monkeys", capital M. At spawn, open your map and head SE. You'll pass a snowy beach and see sugar canes. Head for the sugar canes. You'll find a clay deposit at the foot of a hill about 20 blocks past the canes, in sand.
Starting a new world isn't a big deal, IMO, because you can load saved games pretty quickly. The biggest pain is losing your inventory... but that's easily addressed if you duplicate...