I've made heaps of backgrounds for tanks, for little non structural jobs I use selleys glass silicon, around $9 a tube from Bunnings. It's %100 silicon and its acetic cure (the vinegar smell when drying) so safe to use in your aquariums around 48-72 hrs after use. It's only in clear though maybe someone could point you in the right direction for black
The smiling rock in Wangara sells nice stone as well as wangara sands and soils.
Could I suggest that you maybe get a piece styrofoam and cut it to the size of the back of your tank, this will give you a solid backing to attach pebbles to and also allow you to scoop some foam out to sink stones into or double up for extra layers.
You can also use a black pond sealer paint (cromlins) to touch up any areas in between which adds some depth.
It's a fun project, let us know how you go