No disrespect intended to the West Systems products, but their fillers are very expensive and identical to the stuff you can get elsewhere in bulk. 2oz of West 410 (microlight), which I believe are the white glass micro balloons, is $16. You can get FIVE QUARTS (160 ounces) of the 3M brand glass microballoons for $9.75. Same with the phenolic microballoons (the redish medium weight filler), and cabosil (the fumed silica).

http://www.uscomposites.com/fillers.html