I don't know if this has been mentioned on this thread, yet, but just in case it hasn't, while reaching level 5 gardening then hitting the trailer-like house in Oasis Springs and a bit east of it to get a cutting from a tomato plant and graft it with whatever to harvest tomatoes later has helped, if you have the City Living expansion, it's much easier to get tomatoes. You can actually buy them! One place that I can confirm sells tomatoes, potatoes, and other hard-to-find produce is the "Produce" stand in the art district of San Myshuno town. Hope this helps someone.