No Cab-o-Sil is a different product. What it does is make the epoxy "Thixotropic". In other words it makes it less prone to "run" or drip. It is not lightweight like microbaloons and is used for different purposes. Aircraft Spruce and Specialty sells it to the EAA crowd, and I suspect that the folks that supply composite products to modelers also stock it, but I haven't checked.