Tony,
I buy coarse silica sand from our local builder's supply (not sure of the grade, I will have to look). I use it for my sand filtration system, to clean the irrigation water I pump from the river to use in the gardens/greenhouses. I would think that any builder's supply would have something appropriate, but be aware that this is sold in 80 pound bags. (I think most people use it in concrete mixes)
What is the name of the builder's supply place? 80lb bags are no problem, I burned through the 100lb bag of fine grade stuff like it was nothing.