I love the way that people assume that the guy at the shop must be wrong. I worked at placies (and other stores) for over 5 years and you can not expect them to know all the alternative names for a product.
Yes they sell sand and yes they have white sand but no they do not have silica sand. Silica sand needs to have partucular portions of silica and a lack of polutants (iron ore ect) so they can not sell it as silica sand because it is not tested.
As Aquilam_11 said ask at the trade sales counter (in the timber yard building) or ask one of the yardies they will show you what "builders sand", "plastering sand", "block laying sand", "leveling sand"...(i could keep going but I think the point has been made) they have and you can make your own decision as to what kind of sand you want.