This is easy. Get some rock from the ocean. Any ocean. You will not be able to introduce any where near the amount of life that there is in the ocean if you depend on silly things like bacteria in a bottle or stuff from a LFS.
I also realize many people don't live near the sea. I can't help that but almost everyone can buy live rock from someplace.
I use rocks from New York because thats where I live, but no matter where I lived, if I wanted a natural reef tank where it is "similar" to the ocean, I would add something from the ocean.
I feel my tank is "similar" to the ocean because thats what I do. Of course if you are worried about introducing things that you consider a threat (I don't) then there isn't much you can do because those creatures live in the same place and in the same numbers that beneficial organisms do.
In over fifty years of adding mud, rocks and water from the sea, I don't even remember adding anything that was much of a problem.
Of course, you may want to remove some of the larger things.