Do your research, call around to gravel and sand supply companies.. My LFS gets a very nice ray-safe sand out of a company in Portland, OR.
Also, instead of asking for 'sand'.. (which you will find ALOT of Silica, which is bad bad bad.. and sand compacts if too thick.) you might ask for 'fine grain gravel'.. I had more luck with what I wanted asking for this..
I called a few gravel stores asking for 'sand', and they said 'No'.. my LFS called up and asked if they had a 'fine grain gravel' and they said 'Yes'.. so..
Definitely search for 'Sand' and 'Substrate' here in the Stingray forum.. this has been gone over a million times..