Go to a hardware store (Home Depot, Lowes, maybe even Walmart) and you can find slate like this. Even tile stores or if you have it in your area, landscaping stores. But... right now isn't the season for it. I had a hard enough time trying to find gravel. So, I would say wait until spring and they will be everywhere.
55gPLANTED:Rainbows/Loaches/Garras|30gVIV:Crestie|20gL:Leos|10g:Pearl Fighter|5g:Betta|5gChi:Spark Gourami/Badis|5g:ADF|2.5g:Betta|2.5g:Betta|