I'm new to aquarium forums, but only rusty to the hobby. I LOVE that aquariumadvice has a DYI section. I had to go back through some old picture cause I wanted to add this. I wanted some retaining walls to hold back deeper sections of gravel for better planting. Plus I like the different levels at the bottom of the tank. The commercial ones are about $20 each and generally only about 2 maybe 3 inches high. I know a little about various rocks, enough to know that not all rocks are suitable inside an aquarium. Anyway, you can buy a large bag of pond pebbles at Home Depot for $3-$4 (wash it thoroughly) then get aquarium safe silicone $3-$4. You'll need a little patience since it's like putting a horrible puzzle together and there are moments you'll have to let it sit for an hour to let the silicone set up a bit before adding more rock. Also, make sure you don't use any rocks with sharp edges.
I know there are several posts on here stating which large tubes of silicone is safe (the size you put in a caulk gun).
I bought a small tube at Home Depot (near the glue section next to paint) that stated "aquarium safe" it was a green and black package. If you get this tube, learn from my experience and wrap it in duck tape before you start using it. Otherwise, the metal tube ends up with small holes from squeezing it.