I've never seen a tank with a slate bottom! Thats awesome.
I install slate on the back walls of most all my aquariums as a background.
Now on to airplants/tillandsia
Black jungle supply offers many of them at a reasonable price as well as many other great terrarium plants.
There are also if you search airplants on google many companys that sell them. Haven't tried them.
The tillandsias I had from black jungle came in very healthy but my setups were all too humid for them and they quickly perished.