Actually, in the U.S. this situation is changing (I guess it all
depends
on the perceived threat model of the local authorities.) Tim McVeigh
used
an ammonium nitrate bomb and so large purchases of it are being
passively
monitored, although it is still easy enough to steal and just about
every
farm in the country has enough to make a reasonably sized boom. As
far as
nerve gas goes, this is also fairly easy to make (although this fact
is less
widely known since there are few "how to make Sarin" texts floating
around
the Internet :) Sarin was actually discovered almost accidentally
just
prior to WWII and it is only a few steps removed from insecticides,
anyone
with a decent background in organic chemistry can make it.
Biological
agents are more difficult to come by and that is mostly because after
it
was discovered that the Japanese cult which gassed a subway station
had
been trying to get Anthrax specimens the controls on who gets viral
samples
were tightened up. On the other hand most bio labs are poorly
secured and
a simple break-in or even armed robbery will probably get you what
you need.
Nuclear weapons are the only mass-destruction weapons which require a
large
infrastructure to create. The number of countries which have
admitted to
having chemical weapons now that a ban is in place has proven to be
quite
surprising, and after looking at the list of those countries which
were
suspected of having such weapons it becomes quite apparent just how
easy
they are to make...
jim