I have read that starting your own tomato plants is very simple to do by taking cuttings off of a mother plant. In fact, it seemed too easy. Finally, last spring we tried it with fantastic success.
This tomato plant came up on its own in our rosemary plant (from the compost we used to pot the rosemary). I snipped off 6 cuttings and put them in small pots that were filled with our compost and watered well.
After a few days a couple looked like this photo. We kept up with the watering and left the plants in a north facing window so that they wouldn't get sun shinning on them at this early stage.
It took a few weeks, but this photo shows how well they did (5 did fine, one died).
We kept them in the small pots and continued to water well for about 3 weeks or so, then transplanted to bigger pots.
We had tomatoes by the end of the summer and managed to keep one cherry tomato plant alive and producing most of the winter.