You might want to check out ceramic (pottery)supply stores in your area. Laterite is a type of clay that is sometimes used by potters and ceramicists to add color to clay bodies and glazes. This is also a good source for dolomite and calcium carbonate, both used in glazes.
Be sure to check the chemical make up of the laterite here as they can be quite variable, depending on how and where it was mined. Some laterites found in these places have a high amount of Bauxite (form of aluminum) which can be harmful to your fish. The good thing is that all materials from ceramics supply stores are always analyzed and correctly labelled (or they should be!) so you know exactly what you are getting.
On a side note, here in Hawaii, laterite is everywhere! Just dig down a couple of feet and there it is!!! The Islands are are made of this stuff, among other things... like concrete.
Aaron