You can build your own using strong fencing and wooden or metal posts. Or you can buy pre-made tomato cages.
Simply drive a stake into the ground near your plant and gently tie the stem to the stake using twine or yarn, and repeat as the stems grow.
We live down the road from a very large commercial tomato farm.They use a system called The Florida Weave, which is essentially weaving tomato twine between posts and plants to hold up plants and fruit.
It’s pretty simple to construct- just pound 2 metal t-posts in the ground and stretch your wire fencing between them.