Lowes has these do you think they would work?
American Countryside 0.5-cu ft Marble Chip
http://www.lowes.com/pd_2527-66882-2...ductId=3028435
I would make the container out maybe 2" PVC about 15" long. with screw on fitting at the top so I could refill it.
Or Home Depot on sale $2.50
http://www.homedepot.com/p/Vigoro-0-...0943/202292875
Vigoro 0.5 cu. ft. White Marble