If you have a tomato plant and want more, say for a certain variety, here's a way to do it on the cheap.
When your plant grows a new sideshoot or lateral, wait until it gets about 15 cm long and cut it off with a sharp knife. Give it a trim so that it isn't too leafy on top, then take a plant pot and fill with a good potting mix and thoroughly wet it. Using something as a dibbler, make a hole big enough for the cutting/lateral and plant it in the mix. For the next 10 days or more, keep the cutting moist as it will grow roots and become your new plant.
Here is what your cutting should look like on the top, and on the bottom when it has roots..