It looks like some sort of igneous rock (granite, basalt, etc.). Your best bet would be to go to a local landscape supply place and see what they have that you like.
I just took a walk in our construction yard and found several that would work ranging from purple to black to grayish-green. Just be sure to stay away from limestone as it will raise your pH.
At that note, any rock you pick up from a stone yard should be tested before added to an aquarium.
Place the rocks in a bucket of water for a week and test the water. I bought some rocks I was sure would not harm my tank but they leeched ammonia after testing. It was very minor but still.