Reducing acidity in Northern Indian Cuisine (chana masala)
Perhaps you could try using a smaller kind of tomato, like cherry tomatoes.
These are usually much sweeter in my experience and in general have a lot more to offer in terms of taste, compared to large tomatoes.
A mixture of both would also work if you don't have enough small tomatos at hand, since the sweetness of the cherry tomatoes will counteract the acidity of the larger ones.
If smaller tomatoes aren't an option, I would also go for high quality canned ones, as other people have suggested.