I've been experimenting with a lot of readymade sauces for curries over the last 5 years or so. And, so far, I haven't encountered one that I really love. I almost always prefer to cook curries from scratch.
However, about a month ago, I discovered Tilda Simmer Sauces. It's not quite close to "the real thing" of course. But, it's definitely good enough if you have no time to do a lot of cooking. I would especially recommend the Butter Chicken and Chicken Korma sauces.
One good thing about these Tilda simmer sauces is that you don't need to add anything else apart from the meat and/or vegetables. Some sauces would sometimes require ingredients like cream, coconut milk, or yoghurt.
Anyway, I'm not sure about the availability of this brand worldwide. All I know is that they're easily available from most grocery stores in Oz, like Woolworths.