I use Colloidal Slilica as a thickener when I want strength and abrasion resistance.
But wear a dusk mask when you use it as when it is dry (prior to wetting out in the premixed epoxy) it is very fine and fluffy and not something you want to breathe into your lungs.
I just checked and Jamestown Distributors has some for $7.00 a quart with free shipping.
Also, you will find it in a local store that sells West System epoxy products.
BTW in my humble opinion Micro Balloons are not very abrasive resistant.
Of course, there are lots of other fillers - for example wood flour - graphite - fine dust from sanding - etc.

I hope this is useful,
John VB