Sand is a commodity item that's too heavy to ship long distances, so it tends to be locally sourced. That's why things like "play sand" will very considerably in differing parts of the country: in some places it'll be a nice beige color, in my area it's an ugly dark grey.
It you can't find "pool filter sand" locally because it's out of season, just see if you can get another nice-looking silica sand of a suitable consistancy (pool filter sand is just silica sand of a grit that appropriate for pool filters, in very expensive packaging).
Go a local masonry/hardscape material supply yard; i.e., those places that sell brick, pavers, stone, etc. Ask for a bag of white silica sand, in the coarsest grit they have. A 100 lb. bag will probably go for $10 or so.