Over here laterite is found in small, expensive boxes at fish and aquarium stores. It's a rocky, clay-like substance high in iron. It is used primarily for planting aquatic plants in aquariums, though I don't know the advantage for aquarium plants besides it doesn't break down much.
I had some with merriliana and it seemed to like it. I put in a portion with a standard peat/bark/charcoal mix.
Isn't lava rock also a little high in iron? Not suggesting it a s a sub, but just curious.