Microballoons are simply a fine silica filler. You can get the whole kit and kaboodle (the kit I can find, but what's a kaboodle....anyway?
) from WEST System. The more microballoons you add to a mix, the thicker it gets. Any good marine shop should have it. If it's a small job, you can get the epoxy resin/hardener packets which make the job a whole lot easier and virtually fool-proof.
I can look up the part numbers if you need them, as I keep a repair kit on my service van at all times.