Hey.. Really thanks for answering the question. I actually had went for buying the substrate into the local aquarium, I inquired the same details I asked here and even the owner was not able to explain in proper manner.
Even I found the soil substrate too expensive and instead of buying for the whole tank I bought only around 1 kg of the soil substrate.
I added it carefully in one corner of the tank and place some pebbles to create a wall around the soil. Planted the same kind of plants shown at the right corner of the above picture.
I really hope the plants branch out their roots quickly to hold the soil in its place as I keep finding my goldfish digging the soil in search of food. (P.S. I do feed her regularly!)