Cab-O-Sil isfumed silica.  You can get it at most larger plastics stores.  
Try TAP Plastics and if they don't have it, the Gougeon Brothers WEST system
epoxy has it.  If you have a marine chandlery, they should sell it.  If not,
try West marine (no relation to WEST System) on line.  Finally, you can also
get it at Fiberglass Hawaii in Watsonville, California, near Santa Cruz.  If
you talk to the Gougeon Brothers about it, tell them fumed silica since
Cab-O-sil is a product name. I think they call it "high density filler".

As another tip, I hardly ever use either straight microballoons or straight
Cab-O-Sil.  I generally mix the two.  For non structural fairing where I want
the least weight and most sandability, I go with about 25% fumed silica and
75% balloons.  For a harder putty that is more structurally sound, I go the
opposite.  75% fumed silica and 25% balloons.  I do it approximately and all
by eye and these are by volume.  The reason to mix is that straight balloons
require an awful lot of material to be added since it isn't a particularly
effective thixotrope.  By the time you have added enough balloons to make the
mix thixotropic, it becomes like bread dough and hard to spread.  A small
amount of Cabo and it becomes kind of "silky" and spreads much better.

Likewise, if you make your mix straight Cabo, it gets very gelatenous.  A lot
like Vaseline and kind of elastic and when hard achieves granite like
properties.  A bit of balloons and it spreads much better.

Another rearkable thixotrope is Kevlar pulp.  Basically a fluffed up staple
fiber, it makes flox look like chewing gum.  DuPont can tell you where to get
it.  They sent me a bunch and I'm still using it.  A very small amount goes a
very long way. Forget sanding though.  Once in place, sit back and admire it
'cause that's the way it'll stay!

 By the way, I buy Cab-O-Sil in 50 pound bags which are actually cheap, about
$75 but are about 50 gallons volume.  Balloons are the same way but their
properties are vastly different.  The problem is getting it in small volumes
costs a lot per pound.  And for the real plastics folks, you can order
microballoons in different size spheres.  We generally get an industrial
grade which has a large diameter distribution but there are many uses
including cosmetics with tight tolerances.

Dan Newland
Super ES