I had no idea you could do this! Apparently, you can root tomato plants from a tomato laterals. What’s a tomato lateral? They are the little shoots on the tomato plant that grow between the main stem and leaf.
To root them, you can cut off the lateral and stick it in dirt. It will root and grow. Or you can start them in a glass of water and see the roots develop yourself.
If you did this early enough in the season, you could feasibly buy one plant and get several free plants from it.