They are right, kind of. There is no such thing as "pre-mixed curries". I would recommend finding some food using grocery delivery apps like BigBasket or Blinkit. You will find lots of options for food which you can throw in a pot with water and cook. Give me a short while, and I'll see if I can share any links.
Edit - Rajma (kidney beans) ready to cook. Yellow dal and rice, ready to cook.
You can also show the shopkeepers these items and ask them for any similar product.