The problem with your question is that elephants are not merely part of a ecosystem to be managed or a resource to be managed.
Just as members of the species Homo sapiens hope that objective extraterrestrial observers would consider them to be semi or even fully intelligent beings and thus worthy of being considered people with rights, there are many other species on Earth that might be considered semi or even fully intelligent beings by objective extraterrestrial observers, beings that might deserve some or all of the rights belonging to people.
Until and unless society can be absolutely certain that such beings are not intelligent beings, it is safer to act as if we were certain they are intelligent beings and punish killing them as if it were an act of murder. Because if we do so needlessly, the bad results will be a lot smaller than if we need to do so and don't.
And the three surviving species of elephants are among the species of close to human intelligence that should be treated like people.
So if you are writing a fantasy story, you might have a scene where Emperor Ru-thlyss XIII is attending the laying the foundation stone for his pet project, a literal ivory tower for his university. And suddenly a space time vortex appears and out pour men and armored elephants who scatter the crowd, slaughter the bodyguards, and capture Emperor Ru-thlyss.
Ru-thlyss is told that in the distant future wizards (in his ivory tower, ironically) discovered that elephants are people and learned how to communicate with them. Because elephants still remember the great ivory hunt that decimated them in the time of Ru-thlyss, human and elephant wizards discovered a way to travel back in time to stop Ru-thlyss and his great ivory hunt.
So now they have taken over the Imperial government and will decree the death penalty for any person who dares to kill an elephant or make any object out of ivory. Ex-Emperor Ru-thlyss will be the first person tortured to death under the new law.
All teeth are made of ivory, and in the world today, if you are thinking of a contemporary story, many millions of persons are born every year and many millions of children a few years old lose their baby teeth. In some societies it is common for children to be paid for their baby teeth by the "tooth fairy", and of course a wealthy enough government could buy all the baby teeth in the world.
And of course many millions of people die every year and a wealthy enough government could pay to harvest the teeth of many of them.
Millions of farm animals are slaughtered every year and I am sure that bones and teeth are harvested and put to some economic use. A wealthy government could buy millions of teeth per year.
Walrus ivory was an important historical alternate source of ivory. The government could try capturing, breeding, and raising Walruses in ever increasing numbers for their tusks.
in a fantasy world, wizards could use spells to track all the tusks and all the teeth of all the elephants in the world. Whenever a tooth fell out of an elephant's mouth, or was swallowed and excreted, or a tusk broke off, or an elephant died, wizards would be magically alerted to the location and use spells to obtain the ivory. Maybe the wizards could also use magic to gather no longer needed teeth from sperm whales.
And in a science fictional setting perhaps technology could do most of what I imagined spells doing. Tiny robot "insects" could fly into elephants mouths and attach tiny trackers to teeth and tusks, for example.
A fantasy story could have wizards seek out elephant graveyards where elephants go to die, loaded with the ivory of elephant generations.
And there is prehistoric ivory dug up from dead mammoths in northern lands.
Genetic engineering could probably be used to make millions of farm animals grow long tusks out of their mouths and other body parts.
Ancient Greeks made often gigantic chryselephantine statues of gods, with wooden frames and thin gold sheets molded into clothing and thin ivory sheets molded to form the flesh. It is claimed that the Greek sculptors had techniques to soften ivory tusks and unroll the concentric layers they were composed of to get thin sheets of ivory much wider than the thicknesses of the tusks. They would mold the ivory sheets onto molds to give it shape before the ivory stiffened.
Since ivory is a rare material, and since the ivory tower will be more impressive the more floor space it has, it should be as close to spherical as it can, because a sphere encloses the most volume for the least amount of expensive ivory cladding - I have grave doubts about the structural strength of ivory.
Thus I can imagine an ivory clad building shaped like a hemisphere 500 feet wide and 250 feet high. Of course that might not be towering enough to be an ivory tower.
Or the tower might be a cylinder 500 hundred feet tall and 250 feet high topped by a dome 500 feet wide and 250 feet tall.
Or the cylinder could be 500 feet wide and 500 feet tall, with a 250 foot high dome on top.
Or the cylinder could be 500 feet wide and 750 feet tall, with a 250 foot high dome on top.
Maybe the tower could have a cylinder 250 feet wide and 250 feet tall, supporting a three quarters dome 500 feet wide and 375 feet tall for a total height of 625 feet tall.
Or for a more tower like form the the tower could be a cylinder 100 feet in diameter by 1,000 feet tall.
I have to say that whenever I think of an ivory tower I picture a marble tower, to be precise, the Leaning Tower of Pisa as it would have looked if it never leaned.
Ivory consists mostly of dentin. It should be possible to synthisize dentin, and if can be synthisized it can be done on a massive industrial scale.
Dentin rates approximately 3 on the Mohs scale of mineral hardness.3
And dentist study the properties of human dentin in a tooth environment. But I am not sure where to find the properties of dentin as a construction material.
You should assume that ivory and dentin is only useful as a cladding material and not as a structural material. And to prevent weathering and pollution problems the ivory should be encased in a clear transparent material that protects it from such effects.