Slate is cool for your tank, looks the bomb as well!! You can build some really nice structurs from it using aquarium sealant to stick it together. Just get down your LFS and they will have slate for sale I'm sure. If not, get down to the local DIY/Hardware store and you can either buy lumps of slate from the garden centre, make sure it's natural and not the coloured stuff, and smash it up with a hammer. Or you can buy Natural Slate floor tiles, again make sure they're not treated with anything. I prefer the floor tiles my self as you can build up the layers after you smash them to create some really cool caves.
Hope this helps!!