I think no one will sale a large amount for that, if you really need it, you should have taken a permit first from ministry of Health or elsewhere. Silica is if i am correct is derived from the element or a substance like acid or solvent and in that case, any large amount is questionable by authority, because, some might be used for making something destructive, if you know what i mean, or something use by sniffers??? I am not really sure of what i said, but, one thing is sure, if you need it in big quantity, you should have taken a permission from authority..  

"There's nothing we can do to change the past, if it teaches  you a lesson profit from it then, forget it."