>Unfortunately, there is no inexpensive commercial source for laterite.
>Aquarium Pharmaceuticals used to sell laterite, but it seems to be no
>longer available, so Dupla is the only source.

Thiel Aquatech sells laterite for about $7 per (lb?, kg?).

Also another company sells a clay based substrate additive called "Substrate 
I can't remember their name, but probably someone has heard of them.