Laterites are defined as highly-weathered clay-rich soils
with abnormally high content of iron and aluminum.  They 
ONLY occur in tropical areas.  If I recall correctly, it 
is the clay content, along with the availability of iron, 
that makes laterite useful.  The clay provides binding 
sites for various trace elements and nutrients, making
them more easily available to the plants through their 
roots.  The intense  weathering also serves to wash out 
other, undesirable compounds and organics. 
If the EPA has banned laterite, it is probably because
it might introduce tropical fungus and disease to 
terrestrial plants.  Dupla markets laterite through
some pet shops and via mail order pet suppliers, like
the Pet Warehouse.  Right now they appear to be the
only supplier of "true" laterite.  Other sources I 
have seen discussed (local red dirt, ground up clay
pots) are not true laterite and may not provide all
of it's benefits.  I have worked for a client who
mined natural clay and made red bricks and pots, and
I can assure you that the original clay was colored
with a red pigment and was not laterite.
I just went to a lecture last night on the geology of
Vietnam and the lecturer pointed out that the thick
laterite deposits were favored by the Viet Cong for
their tunnel systems.  Turned out that his US Army
command post had a VC command post 40 feet below it,
and the Army never knew until they were told after
the war.  Amazing!
 - Jack