If you want extra tomato plants there is an alternative to sowing seeds or buying plants. It’s to root new plants from pinched off sideshoots.
The earlier in the season the better to give the plant time to establish itself and mature in time to fruit and ripen.
The technique is to pinch out a sideshoot, put it in glass of water and in a week or two roots should form and appear. Mine took over 3.
This close up picture is probably around 5 weeks as I’m not intending to plant it but the shoot and roots still look really healthy. It’s now become the central feature of my ‘nature table’ windowsill.
I also found this alternative method of rooting sideshoots which looked interesting and one to try next year.

