you can purchase great stuff online, through Amazon. If you do this get the black kind made for ponds. It will cost you about 8 dollars a can. Also, I have yet to figure out how to save a can once you have began to use it. So you will have to make sure when you start you are ready to use the entire can.
Or you can go to Wal-Mart, paint department, and pick up the great stuff. like 3 bucks a can and waterproof it as mentioned above.
I also have a waterfall that came with a filter. I forget who made it. It has an opening at the bottom for the water to enter, it gets filtered and spits out near the top. The top comes off for easy access to the filter. I used it in a firebelly tank years ago. It was different. I'm sure once you place it in the corner of your tank you could manage to build land and river leading to it.