If you are a stay at home mum, the most ideal is to cook your own porridge. I am giving my baby Heinz Rice Cereal, and will cook porridge every other day, when I'm not feeling lazy
Even for adults, home-cooked food is always the best.
If you want to cook porridge, please make sure that you add more water and make the rice is cooked to fine bits. For a start, you may wish to give more of the porridge water with some rice. As your child gets used to it, you can increase the amount of rice and make the consistency thicker.
Recently, I drained off most of the porridge water and used breast milk instead. Not sure if this is advisable.
My daughter has started on cereal/porridge for about 2 weeks now, and she is now able to eat thick porridge.