I have found potatoes in the community garden in Willow Creek. I didn't know about the blackberries, I shall have to go gather some. Thanks for the tips! I don't garden in RL. Despite having several planters and/or farmers in my lineage, I can't keep a houseplant alive. But, I'm finding it rewarding in this game, nonetheless. Much more so than Sims2 or 3, even. :)
ETA: Very recently one of my Sims found tomatoes at the Spice Festival. I tried a second time with another Sim in a different save, but no such luck. Starting a brand new save, however, again I found them at the Spice Festival. There is a few plants behind the person using the BBQ grill.